Problem: If r and s are the roots of the equation ax2+bx+c=0, the value of r21+s21 is:
Answer Choices:
A. b2−4ac
B. 2ab2−4ac
C. c2b2−4ac
D. c2b2−2ac
E. none of these answers
Solution:
Since r21+s21=r2s2r2+s2=(rs)2(r+s)2−2rs, and since
r+s=−ab and rs=ac, we have r21+s21=c2b2−2ac