Question

If a + b + c equal to 7 and a square + b square + c Square equal to 29, then the value of 3ab+3bc+3ca is
a)10. b)20 c)15 d)30

Answer 1

it's answer is 20.
reason is that-
(a+b+c)2whole square= a square +b square + c square+3ab+3c+3ca
7 square = 29+3ab+bc+ca
49=29+3ab+bc+ca
49-29=3ab+bc+ca
20 is ans