If a^2 + b^2 + c^2 =90 and a+ b+ c = 20 .Then ab + bc + ca is
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Step-by-step explanation:
(a+b+c)^2=a^2+b^2+c^2+2(ab+bc+ac)
20^2=90+2(ab+bc+ac)
400-90=2(ab+bc+ac)
ab+bc+ac=310/2
=155
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