If a + b + c = 0 and $a^2 + b^2 + c^2 = 40 $, then what is the value of ab + bc + ca ?

-20

If a + b + c = 0

$a^2 + b^2 + c^2 = 40 $,

Then what is 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

If a + b  = 0

$a^2 + b^2  = 40 $,

Then what is the value of ab = ?

( a + b )² = a² + b² + 2ab

( 0 )² = 40 + 2ab

2ab = -40

ab = -20