Mark M. answered 01/30/24
Retired math prof. Calc 1, 2 and AP Calculus tutoring experience.
Divide the interval [0, 1] into n equal subintervals, each of length Δx = (1-0)/n = 1/n
The right endpoints of the subintervals are 1/n, 2/n, 3/n, ...,n/n=1
Let xi = right endpoint of interval i = i / n
Let f(x) = 3 / (1 + x6).
For each of the n subintervals, multiply f(xi) by Δx to get:
f(x1)Δx + f(x2)Δx + f(x3)Δx + ... f(xn)Δx
= Δx [ f(x1) + f(x2) + ...+f(xn) ] = (1/n)∑(I = 1 to n) f(i /n) = (1/n)∑(I = 1 to n) [ 3 / (1 + (i/n)6)]
The value of the limit of the sum above as n→∞ is the value of:
∫(from 0 to 1) [3 / (1 + x6)] dx