Question

If
$a + b + c = 11$
and
$a ^{2} + b {}^{2} + c {}^{2} = 71$
then
$ab + bc + ca \: =$
​

Answer 1

Step-by-step explanation:

(a+b+c) square = (11) square

a square + b square + c Square + 2 ab + 2ba + 2ca = 121

71 + 2 ( ab + bc + ca) = 121

2( ab + bc + ca) =121 - 71 =50

ab + bc + ca = 50/2 =25

Hence, ab +bc +ca =25