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Question

If  a+b+c=9 and  a2+b2+c2=29,
then 2(ab+bc+ca) is ______.        


A
59
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B
52
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C
63
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D
74
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Solution

The correct option is A 52
Given,  a+b+c=9 and  a2+b2+c2=29.

Using the identity,
( a+b+c)2 =  a2+b2+c2+2ab+2bc+2ca,

92=29+2(ab+bc+ca)

2(ab+bc+ca)=9229
                                = 8129
                                = 52.

Mathematics

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