Again, we cannot help you identify your error or your confusion if you do not show us what you have done (or at the VERY least thought about).
Use the Definition to find an expression for the area under the graph of f as a limit. Do not evaluate the limit.f(x) = x^{2} +, 4 ≤ x ≤ 6
1 + 2x
lim n → ∞
n i = 1