Question

If a 2 + b 2 +c 2 =90 and a+b+c= 20, then find the value of ab+bc+ca

Answer 1

a+b+c = 20

by squaring the both side we get 
                      a2=b2+c2+2(ab+bc+ac) = 400
                      90+2(ab+ba+ac) = 400
                      2(ab+bc+ac) = 310

ans -              ab+ba+ac = 155