Here are a few links to Khan Academy for videos and even some practice MC Questions for polar and Integral as a limit of a Reimann Sum!

Integral as a limit of a Reimann Sum Video:  https://www.khanacademy.org/math/ap-calculus-ab/ab-integration-new/ab-6-3/v/writing-riemann-sum-limit-as-definite-integral

Integral as a limit of a Reimann Sum Practice:  https://www.khanacademy.org/math/ap-calculus-ab/ab-integration-new/ab-6-3/e/the-definite-integral-as-the-limit-of-a-riemann-sum