Question

if a²+b²+c²=35,ab+bc+ca=23,find the value of a+b+c.

Answer 1

using (a+b+c)² = a² + b² + c² + 2ab + 2bc + 2ca.  

(a+b+c)² = a² + b² + c² + 2(ab + bc + ca)

(a+b+c)²=35+ 2(23)

(a+b+c)²= 35 + 46

(a+b+c)²= 81

a+b+c= -9 or +9

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