How does a*b relates to a^2 and b^2 ?

Question

Give positive numbers a, b and c are such that
ab + bc + ca + abc = 4,
What is the value of
1/a+2 + 1/b+2 + 1/c+2 = ?

Answer 1

Here it is:
a*b=[(a+b)^2-(a^2+b^2)]/2