Q59 / 91 · KVPY 2020 (SB/SX) · Mathematics

Let a, b, c be non-zero real roots of the equation x3+ax2+bx+c=0. Then

  • A
    there are infinitely many such triples a, b, c
  • B
    there is exactly one such triple a, b, c
  • Correct answer
    C
    there are exactly two such triples a, b, c
  • D
    there are exactly three such triples a, b, c

Solution

x3+ax2+bx+c=0=(x-a)(x-b)(x-c)

a+b+c=-a

⇒2a+b+c=0 …i

ab+bc+ca=b …ii

abc=-c⇒ab=-1[∵c≠0] …iii

Also a is a root of equation

⇒2a3+ab+c=0⇒2a3-1+c=0

⇒c=1-2a3

from i

2a2+ab+ac=0

2a2-1+a1-2a3=0

2a2-2a4+a-1=0

2a2(1-a)(1+a)+(a-1)=0

⇒(1-a)2a2(a+1)-1=0

⇒a=1 or 2a3+2a2-1=0

when a=1, b=-1a=-1 and c=1-2a3=-1

when 2a3+2a2-1=0

There will be only one real solution of

fx=2x3+2x2-1=0

as f'x=6x2+4x=0⇒x=0,-23

f0·f-23<0

∴ corresponding to this real value of a one triplet
is possible

∴ Exactly two triplets (a,b,c) are possible

Hence, the correct answer is option: 'C'.

More Quadratic Equation Questions