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 )

49  =  a2 + b2 + c2 +  40,    

hence  a2 + b2 + c2  = 49 -40 = 9