(a+b+c)^2
The formula is:
a2+b2+c2+2ab+2bc+2ca
Proof (ignore if you don't eant tue proof )
Now we will learn to expand the square of a trinomial (a + b + c).
Let (b + c) = x
Then (a + b + c)2 = (a + x)2 = a2 + 2ax + x2
= a2 + 2a (b + c) + (b + c)2
= a2 + 2ab + 2ac + (b2 + c2 + 2bc)
= a2 + b2 + c2 + 2ab + 2bc + 2ca
Therefore, (a + b + c)2 = a2 + b2 + c2 + 2ab + 2bc + 2ca