write the indicated sum in sigma notation

7^2/21+ 8^2/21 + 9^2/21 + .... 39^2/21

Find two numbers such that their difference is 18and their product is minimal.Write down your complete solution,including all steps as well as a clear statement of your final answer.You may use the math editor, or, if you prefer, plain text, for example using x^2 (for the square of x), x(x+5)=0 (for a sample equation), and x/(x-3) (for division). Show all your work.