if a^2 + b ^2+ c^2
= 90 and a + b + c =20 then find the value of a b + BC + CA
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rishabh138:
it is right answer thanks
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(a+b+c)^2=a^2+b^2+c^2+2ab+2bc+2ca
given a+b+c=20.squaring on both sides
(a+b+c)^2=20^2
a^2+b^2+c^2+2(ab+bc+ca)=400
90+2(ab+bc+ac)=400
ab+bc+ac=400-90/2
ab+bc+ca=155
given a+b+c=20.squaring on both sides
(a+b+c)^2=20^2
a^2+b^2+c^2+2(ab+bc+ca)=400
90+2(ab+bc+ac)=400
ab+bc+ac=400-90/2
ab+bc+ca=155
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