a+b+c=8 ,a²+b²+c²=26, find ab+bc+ca
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a+b+c=8(square on both sides)
(a+b+c)²=8²
using algebraic identity
a²+b²+c²+2(ab+bc+ca)=64
26+2(ab+bc+ca)=64
ab+bc+ca=(64-26)/2
ab+bc+ca=19
(a+b+c)²=8²
using algebraic identity
a²+b²+c²+2(ab+bc+ca)=64
26+2(ab+bc+ca)=64
ab+bc+ca=(64-26)/2
ab+bc+ca=19
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