If a + b + c = 10 and $a^2 + b^2 + c^2 = 48$,then the value of ab + bc + ca is _______.

26

If a + b + c = 10 and $a^2 + b^2 + c^2 = 48$

then the value of ab + bc + ca = ?

If the number of equations are less than the number of variables then we can put the extra variables according to our choice = 

So here two equations given and three variables are present so put c = 0

a + b  = 10

$a^2 + b^2 = 48$

then the value of ab = 

( a + b )² = a² + b² + 2ab

(10)² = 48 + 2ab

100 - 48 = 2ab

2ab = 52

ab = 26