9. Remove parentheses and simplify (x + y)5



We start with an x5 term, and each succeeding term has one less factor of x and one more factor of y until we get to y5. We get the coefficients from the 5th row of Pascal's triangle.



x5 + 5x4y + 10x3y2 + 10x2y3 + 5xy4 + y5