# what is indeterminate and improper integral?

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### 1 Answer

**Indeterminate integral** of `f(x)` (also called **antiderivative**) is function (more precisely a **set of functions**) such that `F'(x)=f(x)`. It is written as `F(x)=int f(x)dx` or `F(x)=int_a^x f(t)dt`.

The latter is definite integral with variable upper limit while the former is just different way of writing the same.

*Example:* `int x^ndx=x^(n+1)/(n+1)+C` where `C in RR`

**Improper integral** is similar to definite integral `int_a^b f(x)dx` but with one difference. In case of improper integral **one or both limits are infinite** e.g. `int_a^infty f(x)dx`

*Example:* ` `

`int_-infty^infty e^-xdx=lim_(a->infty)int_-a^a e^-xdx=lim_(a->infty)-e^-x|_(-a)^a=`

`lim_(a->infty)(-e^-a+e^a)=0+1=1`

As you can see indefinite integral is set of functions while improper and definite integrals are actually numbers (area under the curve `f(x)`).