If a+b+c=12 and a^2+b^2+c^2=64 find the value of ab+bc+ac

Answers (1)

(a+b+c)^2=a^2+b^2+c^2+2ab+2bc+2ca

now we have 

a+b+c=12

and a^2+b^2+c^2=64

So ab+bc+ca=\frac{(a+b+c)^2-(a^2+b^2+c^2)}{2}=\frac{(12*12)-64}{2}=\frac{80}{2}=40

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