Question

if a+b+c =20 and a²+b²+c²=90 find the value of ab+bc+ac​

Answer 1

Answer:

(a+b+c)² = a²+b²+c²+2(ac+bc+ca)

(20)² = 90+2(ab+bc+ca)

400 = 90+ 2(ab+bc+ca)

400-90 =2(ab+bc+ca)

310 = 2(ab+bc+ca)

.ab+bc+ca = 155