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Question

If a+b+c=10 and a2+b2+c2=38,then (ab+bc+ca) is :

A
62
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B
31
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C
64
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D
32
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Solution

The correct option is B 31
Given, a+b+c=10 and a2+b2+c2=38.
Using the identity ( a+b+c)2=a2+b2+c2+2ab+2bc+2ca,
102=38+2(ab+bc+ca)
2(ab+bc+ca)=10238
(ab+bc+ca)=622=31.

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