g _, j Estimate the area under the graph of f(x) = 5x3 between x = 0 and x = 6 using each nite approximation below. a. A lower sum with two rectangles of equal width b. A lower sum with four rectangles of equal width 0. An upper sum with two rectangles of equal width d. An upper sum with four rectangles of equal width ; a. The estimated area using a lower sum with two rectangles of equal width is D square units. * (Type an integer or a decimal. Simplify your answer.) b. The estimated area using a lower sum with four rectangles of equal width is :l square units. (Type an integer or a decimal. Simplify your answer.) c. The estimated area using an upper sum with two rectangles of equal width is El square units. (Type an integer or a decimal. Simplify your answer.) d. The estimated area using an upper sum with four rectangles of equal width is :] square units. (Type an integer or a decimal. Simplify your answer.)