Provided: a + b + c = 3
Square both sides,
(a + b+c)^2 = 9
Expand,
a^2 + b^2+c^2 + 2(ab + bc + ca) = 9
a^2+b^2+c^2 = 9 - 2(ab + bc + ca) ........................................................[1]
(x-y)^2 \geq 0 \rightarrow x^2 + y^2 \geq 2xy
a^2 + b^2 \geq 2ab \\ \\ ~~~~~~b^2 +...