Question

if a+b+c=13 , a^2+b^2+c^2=69 . then the value of ab+bc+ca is ​

Answer 1

(a+b+c)^2 = a^2+b^2+c^2+2(ab+bc+ca)

13^2= 69+2(ab+bc+ca)

169-69=2(ab+bc+ca)

100/2=50

I. e. ab+bc+ca=50