Video narration
What does (a+b)^2 equal? Assuming the 2 means the whole bracket is squared, start with the multiplication you already know: rewrite it as (a+b)(a+b). The exponent applies to the entire bracket, so you multiply every term in the first bracket by every term in the second. That gives four results: a times a is a^2, a times b is ab, b times a is another ab, and b times b is b^2. Now comes the important turn: the shortcut is not to square a and b separately. The middle term exists because each bracket creates its own ab. Since those two products match, ab plus ab becomes 2ab. Nothing mysterious is being added; it is simply the same bracket multiplication. So the four products simplify to a^2+2ab+b^2. That answers the question: (a+b)^2 equals a^2+2ab+b^2, because multiplying the two matching brackets creates two ab terms.