a+b+c = 9, a²+b²+c² = 29 হলে, ab+bc+ca এর মান কত?

A 52

B 46

C 26

D 22

Correct Answer: Option C

a+b+c = 9, a²+b²+c² = 29, ab+bc+ca = ?

(a+b+c)2 = a²+b²+c² +2(ab+bc+ca)

ab + bc + ca = (a+b+c)2 - (a2 +b2 +c2 )/2

= 92 - 29 /2 = 81 - 29 / 2 = 26