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利用者:ShuBraque/sandbox/二次方程式の解の公式

2次方程式の解の公式

en:代数学基礎において、2次方程式の解の公式とは2次方程式の解を求める式である。因数分解平方完成en:グラフを書くなど解の公式を使う以外にも2次方程式を解くためには方法がある。しかし、2次方程式の解の公式を使うことが最も便利な方法であることがしばしばある。

2次方程式の一般形は

ここで x は未知数を示しており、 abc は定数である(ただし a は0ではない)。2次方程式が2次方程式の解の公式を満たすことを、2次方程式の解の公式を2次方程式に代入することで確かめることができる。 2次方程式の解の公式によって与えられるそれぞれの解はen:根と呼ばれる。

解の公式の導出[編集]

平方完成を理解していれば、2次方程式の解の公式を導出することができる[1][2]。このため、2次方程式の解の公式の導出は学生に課題として与えられることがあり、この課題を行うことで学生はこの重要な公式を再発見することが可能なのである[3][4]。陽関数表示の導出は次の通り。

a が0でないことから、 a で割ることが可能である。2次方程式を a で割る。

c/a を等式の両辺から引く。すると次のようになる。

この2次方程式は平方完成が適用可能な形となっている。よって、等式の両辺に定数を足し、等式の左辺を平方完成とする。

これを変形する。

.

最後に、右辺の項を変形し公分母を得ることで、次の式を得る。

等式が平方完成された。等式の両辺の平方根を取ることで次の式を得る。

x イコールの形に直すことで2次方程式の解の公式を得る。

このプラスマイナス記号 "±" は次の2つを示している。

これらは2次方程式の解である[5]。この導出以外にも様々な導出方法があり、それぞれ多少の違いがあるが、ほとんどのものがの操作に関するものである。

一部の文献、特に古いものでは [6][7]のような異なるパラメータ表示をしていることがあるwhere b has a magnitude one half of the more common one. These result in slightly different forms for the solution, but are otherwise equivalent.

発展の歴史[編集]

ラファエロアテナイの学堂の中のユークリッド

二次方程式に解を与える最初期の方法は幾何学的であった。バビロニアのくさび文字で書かれた文字板には二次方程式を解くことに単純化可能な問題が含まれていた[8]。エジプト中王国の時代 (2050 BC to 1650 BC) にまで遡る、エジプトのen:Berlin Papyrusには二項の二次方程式の解が含まれていた[9]

ギリシャの数学者ユークリッド (およそ 300 BC) は原論という自身の著作のなかで二次方程式を解くのに幾何学的方法を使った。原論は非常に大きな影響を与えた数学の学術文献である[10]。およそ200 BCの中国の九章算術には二次方程式に対する解法が登場する[11][12]。ギリシャの数学者ディオファントス (およそ 250 BC)は、自身の著作算術において二次方程式を解いたが、彼の手法はユークリッドの幾何学的手法と比較してより代数学的であったとされる[10]。ディオファントスの解は、たとえ2つの根が共に正であってもひとつの根のみを与える[13]

インドの数学者であるブラーマグプタ (597–668 AD)は自身の学術論文en:Brāhmasphuṭasiddhāntaの中で明示的に二次方程式の解の公式をを示した。Brāhmasphuṭasiddhāntaは628 ADに出版されたが[14]、記号(symbol)ではなく言葉を使って書かれていた[15]。ブラーマグプタの二次方程式 の解は次の通りである。"To the absolute number multiplied by four times the [coefficient of the] square, add the square of the [coefficient of the] middle term; the square root of the same, less the [coefficient of the] middle term, being divided by twice the [coefficient of the] square is the value."[16]これは次に等しい。

初期のギリシャおよびインドの数学者に影響を受けた9世紀のペルシャの数学者フワーリズミーは、二次方程式を代数学的に解いた[17]。すべてのケースに対して有効な二次方程式の解の公式は1594年にシモン・ステヴィンによって最初に得られた[18]。1637年にはルネ・デカルトによってen:La Géométrieが出版されたが、この本には今日私たちが知っている形式で二次方程式の解の公式が収録されている。一般解が現代的な数学の学術文献に初めて登場したのは1896年で、Henry Heatonによる論文のなかで言及されたものである[19]

Importance of this solution[編集]

Among the many equations that one encounters while studying algebra, the quadratic formula is one of the most important, and is considered the most useful method of solving quadratic equations.[20][21] Unlike some other solution methods such as factoring, the quadratic formula can be used to solve any quadratic equation.[22][23] Many equations that do not initially appear to be quadratic can be put into quadratic form, and solved using the quadratic formula.[24] For these reasons, it is often memorized.[25][26]

Completing the square also allows for the solution of all quadratics, as it is mathematically equivalent, but the quadratic formula gives a result without the need for so much algebraic manipulation. As such, it is generally considered more practical to use the formula.[23][27][28][29] Completing the square is very useful for other purposes, such as putting the equations for conic sections into standard form.[30]

Other derivations[編集]

A number of alternative derivations of the quadratic formula can be found in the literature. These derivations either (a) are simpler than the standard completing the square method, (b) represent interesting applications of other frequently used techniques in algebra, or (c) offer insight into other areas of mathematics.

Alternate method of completing the square[編集]

The great majority of algebra texts published over the last several decades teach completing the square using the sequence presented earlier: (1) divide each side by a, (2) rearrange, (3) then add the square of one-half of b/a.

As pointed out by Larry Hoehn in 1975, completing the square can be accomplished by a different sequence that leads to a simpler sequence of intermediate terms: (1) multiply each side by 4a, (2) rearrange, (3) then add .[31]

In other words, the quadratic formula can be derived as follows:

This actually represents an ancient derivation of the quadratic formula, and was known to the Hindus at least as far back as 1025 AD.[32] Compared with the derivation in standard usage, this alternate derivation is shorter, involves fewer computations with literal coefficients, avoids fractions until the last step, has simpler expressions, and uses simpler math. As Hoehn states, "it is easier 'to add the square of b' than it is 'to add the square of half the coefficient of the x term'".[31]

By substitution[編集]

Another technique is solution by substitution. In this technique, we substitute into the quadratic to get:

Expanding the result and then collecting the powers of produces:

We have not yet imposed a second condition on and , so we now choose m so that the middle term vanishes. That is, or . Subtracting the constant term from both sides of the equation (to move it to the right hand side) and then dividing by a gives:

Substituting for gives:

Therefore ; substituting provides the quadratic formula.

By using algebraic identities[編集]

Let the roots of the standard quadratic equation be and . At this point, we recall the identity:

Taking square root on both sides, we get

Since the coefficient a ≠ 0, we can divide the standard equation by a to obtain a quadratic polynomial having the same roots. Namely,

From this we can see that the sum of the roots of the standard quadratic equation is given by , and the product of those roots is given by

Hence the identity can be rewritten as:

Now,

Since, , if we take then we obtain and if we instead take then we calculate that Combining these results by using the standard shorthand, we have that the solutions of the quadratic equation are given by:

By Lagrange resolvents[編集]

An alternative way of deriving the quadratic formula is via the method of Lagrange resolvents, which is an early part of Galois theory.[33] This method can be generalized to give the roots of cubic polynomials and quartic polynomials, and leads to Galois theory, which allows one to understand the solution of algebraic equations of any degree in terms of the symmetry group of their roots, the Galois group.

This approach focuses on the roots more than on rearranging the original equation. Given a monic quadratic polynomial

assume that it factors as

Expanding yields

where and .

Since the order of multiplication does not matter, one can switch and and the values of p and q will not change: one says that p and q are symmetric polynomials in and . In fact, they are the elementary symmetric polynomials – any symmetric polynomial in and can be expressed in terms of and The Galois theory approach to analyzing and solving polynomials is: given the coefficients of a polynomial, which are symmetric functions in the roots, can one "break the symmetry" and recover the roots? Thus solving a polynomial of degree n is related to the ways of rearranging ("permuting") n terms, which is called the symmetric group on n letters, and denoted For the quadratic polynomial, the only way to rearrange two terms is to swap them ("transpose" them), and thus solving a quadratic polynomial is simple.

To find the roots and consider their sum and difference:

These are called the Lagrange resolvents of the polynomial; notice that one of these depends on the order of the roots, which is the key point. One can recover the roots from the resolvents by inverting the above equations:

Thus, solving for the resolvents gives the original roots.

Formally, the resolvents are called the discrete Fourier transform (DFT) of order 2, and the transform can be expressed by the matrix with inverse matrix The transform matrix is also called the DFT matrix or Vandermonde matrix.

Now is a symmetric function in and so it can be expressed in terms of p and q, and in fact as noted above. But is not symmetric, since switching and yields (formally, this is termed a group action of the symmetric group of the roots). Since is not symmetric, it cannot be expressed in terms of the polynomials p and q, as these are symmetric in the roots and thus so is any polynomial expression involving them. Changing the order of the roots only changes by a factor of and thus the square is symmetric in the roots, and thus expressible in terms of p and q. Using the equation

yields

and thus

If one takes the positive root, breaking symmetry, one obtains:

and thus

Thus the roots are

which is the quadratic formula. Substituting yields the usual form for when a quadratic is not monic. The resolvents can be recognized as being the vertex, and is the discriminant (of a monic polynomial).

A similar but more complicated method works for cubic equations, where one has three resolvents and a quadratic equation (the "resolving polynomial") relating and which one can solve by the quadratic equation, and similarly for a quartic (degree 4) equation, whose resolving polynomial is a cubic, which can in turn be solved. The same method for a quintic equation yields a polynomial of degree 24, which does not simplify the problem, and in fact solutions to quintic equations in general cannot be expressed using only roots.

See also[編集]

References[編集]

  1. ^ Rich, Barnett; Schmidt, Philip (2004), Schaum's Outline of Theory and Problems of Elementary Algebra, The McGraw–Hill Companies, ISBN 0-07-141083-X, http://books.google.com/?id=8PRU9cTKprsC , Chapter 13 §4.4, p. 291
  2. ^ Li, Xuhui. An Investigation of Secondary School Algebra Teachers' Mathematical Knowledge for Teaching Algebraic Equation Solving, p. 56 (ProQuest, 2007): "The quadratic formula is the most general method for solving quadratic equations and is derived from another general method: completing the square."
  3. ^ Rockswold, Gary. College algebra and trigonometry and precalculus, p. 178 (Addison Wesley, 2002).
  4. ^ Beckenbach, Edwin et al. Modern college algebra and trigonometry, p. 81 (Wadsworth Pub. Co., 1986).
  5. ^ Sterling, Mary Jane (2010), Algebra I For Dummies, Wiley Publishing, p. 219, ISBN 978-0-470-55964-2, http://books.google.com/?id=2toggaqJMzEC&pg=PA219&dq=quadratic+formula#v=onepage&q=quadratic%20formula&f=false 
  6. ^ Kahan, Willian (November 20, 2004), On the Cost of Floating-Point Computation Without Extra-Precise Arithmetic, http://www.cs.berkeley.edu/~wkahan/Qdrtcs.pdf 2012年12月25日閲覧。 
  7. ^ “Quadratic Equation”, Proof Wiki, http://www.proofwiki.org/wiki/Quadratic_Equation#Also_defined_as 2012年12月25日閲覧。 
  8. ^ Irving, Ron (2013). Beyond the Quadratic Formula. MAA. p. 34. ISBN 978-0-88385-783-0. http://books.google.com/books?id=CV_UInCRO38C&pg=PA39 
  9. ^ The Cambridge Ancient History Part 2 Early History of the Middle East. Cambridge University Press. (1971). p. 530. ISBN 978-0-521-07791-0. http://books.google.com/books?id=slR7SFScEnwC&pg=PA530 
  10. ^ a b Irving, Ron (2013). Beyond the Quadratic Formula. MAA. p. 39. ISBN 978-0-88385-783-0. http://books.google.com/books?id=CV_UInCRO38C&pg=PA39 
  11. ^ Aitken, Wayne. “A Chinese Classic: The Nine Chapters”. Mathematics Department, California State University. 2013年4月28日閲覧。
  12. ^ Smith, David Eugene (1958). History of Mathematics. Courier Dover Publications. p. 380. ISBN 978-0-486-20430-7. http://books.google.com/books?id=uTytJGnTf1kC&pg=PA380 
  13. ^ David Eugene Smith (1958). "History of mathematics". Courier Dover Publications. p.134. ISBN 0-486-20429-4
  14. ^ Bradley, Michael. The Birth of Mathematics: Ancient Times to 1300, p. 86 (Infobase Publishing 2006).
  15. ^ Mackenzie, Dana. The Universe in Zero Words: The Story of Mathematics as Told through Equations, p. 61 (Princeton University Press, 2012).
  16. ^ Stillwell, John (2004). Mathematics and Its History (2nd ed.). Springer. p. 87. ISBN 0-387-95336-1 
  17. ^ Irving, Ron (2013). Beyond the Quadratic Formula. MAA. p. 42. ISBN 978-0-88385-783-0. http://books.google.com/books?id=CV_UInCRO38C&pg=PA39 
  18. ^ Struik, D. J.; Stevin, Simon (1958), The Principal Works of Simon Stevin, Mathematics, II-B, C. V. Swets & Zeitlinger, p. 470, http://www.dwc.knaw.nl/pub/bronnen/Simon_Stevin-%5bII_B%5d_The_Principal_Works_of_Simon_Stevin,_Mathematics.pdf 
  19. ^ Heaton, H. (1896) A Method of Solving Quadratic Equations, American Mathematical Monthly 3(10), 236–237.
  20. ^ Jahr, Cathy. Barron's How to Prepare for the Tennessee Gateway High School Exit Exam in Algebra, p. 137 (Barron's Educational Series, 2005): "The Quadratic Formula is one of the most important formulas in mathematics because it is a method for solving all quadratic equations."
  21. ^ Heywood, Arthur. Intermediate algebra: lecture-lab, p. 235 (Dickenson Pub. Co., 1975): "The quadratic formula is one of the most important formulas in mathematics, and we will now spend some time studying many different ways of using it."
  22. ^ Blanton, Floyd. Modern College Algebra, p. 162 (McGraw–Hill, 1967): "The quadratic formula is the most powerful method for solving quadratics since it can be used to solve any quadratic."
  23. ^ a b Smith, R. and Peterson, J. Introductory Technical Mathematics, pp. 408–409 (Cengage Learning 2006): "The factoring method has limited application. Only certain quadratic equations can be solved by factoring. Completing the square…can be a rather long and complicated procedure and is seldom used in practical applications. [The] quadratic formula…is the most useful method for solving complete quadratic equations."
  24. ^ Banks, John. Elements of Algebra, p. 97 (Allyn and Bacon, 1962): "The quadratic formula is one of the most useful formulas in elementary mathematics. You should be certain you know what it is and how to use it. Many other equations can be solved by first reducing them to quadratic form."
  25. ^ Larson, R. and Hodgkins A. College Algebra with Applications for Business and Life Sciences, p. 104 (Cengage Learning 2009): "The Quadratic Formula is one of the most important formulas in algebra, and you should memorize it."
  26. ^ McConnell, John. Algebra, p. 603 (Scott Foresman 1993): "The Quadratic Formula is one of the most famous formulas in all of mathematics. You should memorize it today."
  27. ^ Payne, M. Intermediate Algebra, p. 289 (West Publishing 1985): "While the method of completing the square may be used to solve quadratic equations, it is more involved than the quadratic formula, and is seldom used in practical work."
  28. ^ Davis, L. Technical Mathematics, p. 174. (Merrill Publishing 1990): "You can use the quadratic formula, as well as completing the square, to solve any quadratic equation. However, you will find that the quadratic formula is easier to use."
  29. ^ Dugopolski, Mark. Algebra for College Students, p. 541 (McGraw Hill 2006): "Any quadratic equation can be solved by completing the square or using the quadratic formula. Because the quadratic formula is usually faster, it is used more often than completing the square."
  30. ^ Sterling, Mary. CliffsStudySolver: Algebra II, p. 60 (Houghton Mifflin Harcourt 2012).
  31. ^ a b Hoehn, Larry (1975). “A More Elegant Method of Deriving the Quadratic Formula”. The Mathematics Teacher 68 (5): 442–443. 
  32. ^ Smith, David E. (1958). History of Mathematics, Vol. II. Dover Publications. p. 446. ISBN 0486204308 
  33. ^ Prasolov, Viktor; Solovyev, Yuri (1997), Elliptic functions and elliptic integrals, AMS Bookstore, ISBN 978-0-8218-0587-9, http://books.google.com/?id=fcp9IiZd3tQC , §6.2, p. 134

External links[編集]