Te dejo el 4 corregido
$$\begin{align}&\\&\int_{-2}^6 (5x + \frac{2}{3}) dx\\&a=-2\\&b=6\\&\Delta x = \frac{b-a}{n}=\frac{8}{n}\\&x_i=a + i \Delta x = -2 + \frac{8i}{n}\\&f(x_i) = 5x_i+\frac{2}{3} = 5 (-2+\frac{8i}{n})+\frac{2}{3}=\frac{40i}{n}-\frac{28}{3}\\&Retomando...\\&\int_{-2}^6 (5x + \frac{2}{3}) dx = \lim_{n \to \infty} \sum_{i=1}^{n} f(x_i) \Delta x = \\&\lim_{n \to \infty} \sum_{i=1}^{n} (\frac{40i}{n}-\frac{28}{3} ) \cdot \frac{8}{n} = \\&\lim_{n \to \infty} \sum_{i=1}^{n} \frac{320i}{n^2}-\frac{224}{3n} =\\&\lim_{n \to \infty} \bigg( \sum_{i=1}^{n} \frac{320i}{n^2}- \sum_{i=1}^{n} \frac{224}{3n}\bigg) =\\&\lim_{n \to \infty} \bigg(\frac{320}{n^2} \sum_{i=1}^{n} i- \frac{224}{3n} \sum_{i=1}^{n} 1 \bigg) =\\&\lim_{n \to \infty} \bigg(\frac{320}{n^2} \frac{n(n+1)}{2}- \frac{224}{3n} n \bigg) =\\&\lim_{n \to \infty} \bigg(\frac{160n^2+160n}{n^2} - \frac{224}{3} \bigg) \\&\text{ (cuando n} \to \infty)\\&\to 160 - \frac{224}{3} =\frac{256}{3}\end{align}$$
El ejercicio 2 y 3 los tienes AQUI
Salu2