# Math Help - Improper Integrals

1. ## Improper Integrals

Evaluate the integral or state that it diverges:

integral from 0 to 4 of dr/sqrt(4-r)

2. Originally Posted by smartartbug
Evaluate the integral or state that it diverges:

integral from 0 to 4 of dr/sqrt(4-r)
$\int_0^4 \frac{1}{ \sqrt{4-r}}dr = -ln(4-r)_0^4 = -(ln(0) - ln(4))$

note that $ln(0)$ does not exist but

$\lim_{x \to 0 } ln(x) = - \infty$

Therefore this integral diverges

3. Originally Posted by AllanCuz
$\int_0^4 \frac{1}{ \sqrt{4-r}}dr = -ln(4-r)_0^4 = -(ln(0) - ln(4))$

note that $ln(0)$ does not exist but

$\lim_{x \to 0 } ln(x) = - \infty$

Therefore this integral diverges
lolwut...?

$\int_0^4 \frac{1}{ \sqrt{4-r}}dr$

let $u=4-r$ therefore $du=(-1)dx$

$\int_4^0 \frac{1}{ \sqrt{u}}(-1)du$

$-\int_4^0 \frac{1}{ \sqrt{u}}du$

$\int_0^4u^{-\frac{1}{2}}du$

4. Originally Posted by DrDank
lolwut...?

$\int_0^4 \frac{1}{ \sqrt{4-r}}dr$

let $u=4-r$ therefore $du=(-1)dx$

$\int_4^0 \frac{1}{ \sqrt{u}}(-1)du$

$-\int_4^0 \frac{1}{ \sqrt{u}}du$

$\int_0^4u^{-\frac{1}{2}}du$
oh **** I missed the square root...don't drink and integrate!