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)² = a² + b² + c² + 2 (ab + bc + ca)
(7)² = a² + b² + c² + 2 (20)  ⇒  a² + b² + c² = 49 - 40 = 9

hope this helps

Answer 2

(a+b+c)^2= (a^2+b^2+c^2) +2ab + bc+ca
(7)^2 = a^2+b^2+c^2+ 2(ab+bc+ca)
49 = a^2+b^2+c^2 +2(20)
49-40=9 = a^2+b^2+c^2

I hope that it's right