left, midpoint, and right riemann sums were used to estimate the area between the graph of f(x) and the x-axis on the interval [3,7]. we know that f is a function such that f(x)>0 and f'(x)<0 on the interval [3,7]. the same number of subintervals were used to produce the approximations. the estimates were 1.345, 1.578 and 1.723. which rule produces each estimate? justify your answer.