If a+b+c=11 and a2+b2+c2=81,find ab+bc+ca
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Answer:
a+b+c=11 & 2a+2b+2c= 81
we know ,
(a+b+c)^2 = 2a+2b+2c +ab+BC+CA
=(11)^2= 81 +ab+bc+ca
=121 = 81+ab+bc+ca
= 121-81 =ab+bc+ca
=40=ab+bc+ca
.
. . ab+bc+ca=40
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