Expansions-4

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If a 2 + b 2 + c 2 = 29 and a + b + c = 9, find : a b + b c + ca

Solution:

Since, (a + b + c) 2 = a 2 + b 2 + c 2 + 2 ( a b + b c + c a )

Therefore, ( 9 ) 2 = 29 + 2 ( a b + b c + c a )

=> 81 - 29 = 2 ( a b + b c + c a )

Therefore, ( a b + b c + c a ) = 52 / 2 = 26

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