if a ²+ b² + c² =80 and a + b + C = 20 then find the value of ab+bc +ca
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a+b+c=20
squaring on both sides
(a+b+c)square =400
a square+b square +c square+2ab+2bc+2ca=400
given asquare +bsquare+csquare=80
80+2(ab+bc+ca) =400
2(ab+bc+ca)=320
ab+bc+ca=160
squaring on both sides
(a+b+c)square =400
a square+b square +c square+2ab+2bc+2ca=400
given asquare +bsquare+csquare=80
80+2(ab+bc+ca) =400
2(ab+bc+ca)=320
ab+bc+ca=160
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