Question

If a+b+c=7 and ab+bc+ca=20 then find the value of a 2 +b 2 +c 2 .

Answer 1

a+b+c = 7

(a+b+c)2 = (7)2

 a2+b2+c2+2ab+2bc+2ac  = 49

 a2+b2+c2+2(ab+bc+ca) = 49

a2+b2+c2+2x20 = 49

a2+b2+c2+40=49

a2+b2+c2=49-40

a2+b2+c2=9