Question

If p, q, r are roots of the equation x^3 – 3x + 7 = 0 then the value of (p^3 + q^3 + r^3) is : (A) 7 (B) –21 (C) 21 (D) –7

Answer 1

$\huge\boxed{Answer}$

Given :-

p , q & r are the roots of the cubic

equation - x³ - 3x + 7 = 0

From comparing the standard form of

cubic eq. ax³ + bx² + cx + d = 0

Here , a = 1 , b = 0 , c = -3 , d = 7

then ,

p + q + r = -b/a = 0

Here , p + q + r = 0

So , p³ + q³ + r³ = 3pqr

& pqr = -d/a = -7

Now ,

p³ + q³ + r³ = 3(-7)

p³ + q³ + r³ = -21