Department of Pre-University Education, KarnatakaPUC Karnataka Science Class 11

# If a and B Are the Roots of X2 − 3x + P = 0 and C, D Are the Roots X2 − 12x + Q = 0, Where A, B, C, D Form a G.P. Prove that (Q + P) : (Q − P) = 17 : 15. - Mathematics

If a and b are the roots of x2 − 3x + p = 0 and c, d are the roots x2 − 12x + q = 0, where a, b, c, d form a G.P. Prove that (q + p) : (q − p) = 17 : 15.

#### Solution

We have,
a +b = 3, ab = p, c + d =12 and cd = q
a, b, c and d form a G.P.
∴ First term = a,  b = ar, c = ar2 and d = ar3
Then, we have
a + b = 3  and c + d = 12

$\Rightarrow a + ar = 3$

$\Rightarrow a( 1 + r ) = 3 . . . \left( i \right)$

$\text { Similarly, } a r^2 (1 + r) = 12 . . . \left( ii \right)$

$\Rightarrow \frac{a r^2 \left( 1 + r \right)}{a\left( 1 + r \right)} = \frac{12}{3}$

$\Rightarrow r^2 = 4$

$\Rightarrow r = 2$

$\therefore a \left( 1 + r \right) = 3$

$\Rightarrow a = 1$

$\text { Now }, p = ab$

$\Rightarrow p = a \times ar = 2$

$\text { And, } q = cd$

$\Rightarrow q = a r^2 \times a r^3 = 2^5 = 32$

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

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#### APPEARS IN

RD Sharma Class 11 Mathematics Textbook
Chapter 20 Geometric Progression
Exercise 20.3 | Q 16 | Page 28