quad sums -- part 2

2 > If a , b are the roots of x2 + px + q = 0 , and they are also the roots of the eqn.

x2n + pn xn + qn = 0 , then prove that a / b and b / a are the roots of

the eqn. xn + 1 + ( x + 1 )n whenever n is an even integer , and vice versa .

1 Answers

23
qwerty ·

a+b = - p (1)

ab = q (2)

a,b are roots of x2n + pn xn + qn = 0

thus a2n + pn an + qn = 0

i.e
(an)2 + pn an + qn = 0

thus we can say that an is a root of the quadratic eqn

x2 + pnx+qn=0

similarly bn is also a root of this eqn

thus an+bn= -pn

anbn = qn

now consider xn + 1 + ( x + 1 )n

put x = b/a and take LCM

thus
\frac{b^{n}}{a^{n}}+1+(\frac{b+1}{a})^{n}

=\frac{b^{n}+ a^{n} +(b+a)^{n}}{a^{n}}

= \frac{-p^{n} + p^{n}}{a^{n}}

(since an+bn = -pn

and a+b = -p

and thus (a+b)n = pn since n = even integer )

=0
similarly we can get the eqn = 0 for x = a/b

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