If a and b are the roots of are roots of ,
Question:

If $a$ and $b$ are the roots of $x^{2}-3 x+p=0$ and $c, d$ are roots of $x^{2}-12 x+q=0$, where $a, b, c, d$, form a G.P. Prove that $(q+p):(q-p)=17: 15$.

 

Solution:

It is given that $a$ and $b$ are the roots of $x^{2}-3 x+p=0$

$\therefore a+b=3$ and $a b=p \ldots$ (1)

Also, $c$ and $d$ are the roots of $x^{2}-12 x+q=0$

$\therefore c+d=12$ and $c d=q \ldots$ (2)

It is given that $a, b, c, d$ are in G.P.

Let $a=x, b=x r, c=x r^{2}, d=x r^{3}$

From (1) and (2), we obtain

$x+x r=3$

$\Rightarrow x(1+r)=3$

$x r^{2}+x r^{3}=12$

$\Rightarrow x r^{2}(1+r)=12$

On dividing, we obtain

$\frac{x r^{2}(1+r)}{x(1+r)}=\frac{12}{3}$

$\Rightarrow r^{2}=4$

$\Rightarrow r=\pm 2$

When $r=2, x=\frac{3}{1+2}=\frac{3}{3}=1$

When $r=-2, x=\frac{3}{1-2}=\frac{3}{-1}=-3$

Case I:

When $r=2$ and $x=1$,

$a b=x^{2} r=2$

 

$c d=x^{2} r^{5}=32$

$\therefore \frac{q+p}{q-p}=\frac{32+2}{32-2}=\frac{34}{30}=\frac{17}{15}$

i.e., $(q+p):(q-p)=17: 15$

Case II:

When $r=-2, x=-3$,

$a b=x^{2} r=-18$

$c d=x^{2} r^{5}=-288$

$\therefore \frac{q+p}{q-p}=\frac{-288-18}{-288+18}=\frac{-306}{-270}=\frac{17}{15}$

i.e., $(q+p):(q-p)=17: 15$

Thus, in both the cases, we obtain $(q+p):(q-p)=17: 15$

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