Estimate the area under the graph of x}: 4x3 between at: O and x: 6 using each finite approximation below. a. A lower sum with two rectangles of equal width b. A lower sum with tour rectangles of equal width c. An upper sum with two rectangles of equal width cl. An upper sum with four rectangles of equal width at. 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.) Using rectangles whose height is given by the value of the function at the midpoint of the rectangle's base, estimate the area under the graph using first two and then four rectangles. f(x) = x" between x = 2 and x = 4 Using two rectangles to estimate, the area under f(x) is approximately (Type an integer or a simplified fraction.)Use a finite sum to estimate the average value of f on the given interval by partitioning the interval into four subintervals of equal length and evaluating f at the subinterval midpoints. f(x) = x on [2,4] The average value is . (Type an integer or a simplified fraction.)