Evaluate `int_1^4(x^2-\ x)\ dx` as a limit of sums.

Explanation

Here the function is `(x^2−x)` is continuous in `[1,4]`
since dividing the n length of `[1,4]` in the same length, the length of each subset is h

i.e., `h=(b−a)/n=(4−1)/n=3/n`

Here `a=1b=4`

Now `f(a+ih)=f(1+ih)`

`=(1+ih)^2−(1+ih)`

`=i^2h^2+ih`

By definition
`∫_1^4(x^2−x)dx=lim_(n→∞)(3/n)∑_(i=1)^n(i^2h^2+ih)`


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