What's the difference between radius and diameter?

Use sigma notation to represent the following series for 9 terms.

What is the value of p in the equation y^ = -4x?

I'm at a bookstore with $\$35$ in my pocket and a $20\%$-off coupon. what is the cost of the most expensive book i can buy with my money and the coupon? (assume that there are books of every possible price in the bookstore. it's a large bookstore! )