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If α and β are the zeroes of the polynomial f(x)=ax2+bx+c = then the value of 1/α2 + 1/β2 will be:
Question

If α and β are the zeroes of the polynomial f(x)=ax2+bx+c = then the value of 1/α2 + 1/β2 will be:

A.

b2+2aca2\frac{b^2+2ac}{a^2}

B.

b22acc2\frac{b^2 - 2ac}{c^2}

C.

b22aca2\frac{b^2 - 2ac}{a^2}

D.

b2+2aca2\frac{b^2 + 2ac}{a^2}

Correct option is B

Given:

If α and β are the zeroes of the polynomial f(x) = ax2+bx+c 

Formula used:

Sum of α and β = ba\frac{-b}{a}

Product of α and β = ca\frac{c}{a}

Solution:

Sum of α and β = ba\frac{-b}{a}

Product of α and β = ca\frac{c}{a}

Now,

1α2+1β2=α2+β2α2β2\frac{1}{\alpha^2} + \frac{1}{\beta^2} = \frac{\alpha^2 + \beta^2}{\alpha^2 \beta^2}

Add and substract (2αβ)

α2+β2+2αβ2αβα2β2 \frac{\alpha^2 + \beta^2 + 2\alpha \beta - 2\alpha \beta}{\alpha^2 \beta^2}​​

=(α+β)22(αβ)(αβ)2= \frac{(\alpha + \beta)^2 - 2(\alpha \beta)}{(\alpha \beta)^2}

Put the values, then

=(ba)22(ca)(ca)2= \frac{\left(\frac{-b}{a}\right)^2 - 2 \left(\frac{c}{a}\right)}{\left(\frac{c}{a}\right)^2}

=b2a22(ca)c2a2= \frac{\frac{b^2}{a^2} - 2\left(\frac{c}{a}\right)}{\frac{c^2}{a^2}}

(b22aca2)×a2c2\left(\frac{b^2 - 2ac}{a^2}\right)\times{\frac{a^2}{c^2}}​​

=b22acc2= \frac{b^2 - 2ac}{c^2}

So, the value of 1/α2+ 1/β2will be b22acc2 \frac{b^2 - 2ac}{c^2}.

Thus, the correct answer is (b).


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