Everyone is confused here because you do not know the terms that are used in English for things.
[MATH](a + b)[/MATH] is a binomial expression. "Bi" from the Latin for "twice", "expression" meaning an instruction on what calculation to make. An equation is a statement that two expressions have the same numeric value.
[MATH](a + b + c)[/MATH] is a trinomial expression. "Tri" from the Latin (or possibly Greek) for three.
[MATH](a + b)^2 = a^2 + 2ab + b^2[/MATH] is a valid equation.
No one understands whether you are asking about
[MATH](a + b)^3,\ (a + b + c)^2,\ \text { or possibly even } (a + b + c)^3[/MATH]
There is a well known, general formula for the expansion of the positive integer powers of a binomial.
[MATH]\{a + (\pm 1) * b\}^n = \left ( \sum_{j=0}^n \dbinom{n}{j} * a^{(n-j)} * (\pm 1)^j * b^j \right ),[/MATH]
[MATH]\text {where } \dbinom{n}{j} = \dfrac{n!}{j! * (n - j)!} \text { and is called the binomial coefficient}[/MATH]
because it is a coefficient in the expansion of the power of a binomial.
For an example
[MATH](a - b)^3 = \{a + (-1) * b\}^3 = \left ( \sum_{j=0}^3 \dbinom{n}{j} * a^{(n-j)} * (-1)^j * b^j \right ) = [/MATH]
[MATH]\dfrac{3!}{0! * (3 - 0)!} * a^3 * (-1)^0 * b^0 + \dfrac{3!}{1! * (3 - 1)!} * a^2 * (-1)^1 * b^1 +[/MATH]
[MATH] \dfrac{3!}{2! * (3 - 2)!} * a^1 * (-1)^2 * b^2 + \dfrac{3!}{3! * (3 - 3)!} * a^0 * (-1)^3 * b^3 =[/MATH]
[MATH]\dfrac{6}{1 * 6}a^3(1)(1)+ \dfrac{6}{1 * 2}a^2(-1)b + \dfrac{6}{2 * 1}a(1)b^2 + \dfrac{6}{1 *6}(1)(-1)b^3 = a^3 - 3a^2b + 3ab^2 - b^3.[/MATH]
