$($a$)$ Using the Riemann sums, approximate ${\displaystyle \underset{a}{\overset{b}{}}}f(x)dx$ by choosing $u_i($either $x_i$ or $x_{i-1})$ as the left endpoint of each subinterval. Solve by partitioning the interval $a,b$ into subintervals $-4,-1$

$-1,0$

$0,1$

$1,3$

$3,5$

$5,6.$

${\displaystyle \underset{a}{\overset{b}{}}}f(x)dx$~~

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$($b$)$ Using the Riemann sums, approximate ${\displaystyle \underset{a}{\overset{b}{}}}f(x)dx$ by choosing $u_i($either $x_i$ or $x_{i-1})$ as the right endpoint of each