# confirmation in regards to this question

• Jun 4th 2008, 07:36 AM
ArTiCK
confirmation in regards to this question
Hi all,

I recently posted 'want some confirmation' and it was in relation this question:

dy/dx = (5x^2)(cos^2(y)) for y(1) = 5*pi

i get y = arctan[ 5/3 (x^3 -1)]

however the answer states y = arctan[5/3 (x^3 -1)] + 5*pi...

I have no idea where that 5*pi is coming from hence it lead to the post earlier.

ArTiCk
• Jun 4th 2008, 07:51 AM
CaptainBlack
Quote:

Originally Posted by ArTiCK
Hi all,

I recently posted 'want some confirmation' and it was in relation this question:

dy/dx = (5x^2)(cos^2(y)) for y(1) = 5*pi

i get y = arctan[ 5/3 (x^3 -1)]

however the answer states y = arctan[5/3 (x^3 -1)] + 5*pi...

I have no idea where that 5*pi is coming from hence it lead to the post earlier.

ArTiCk

Assume that:

$y(x) = \arctan[ (5/3) (x^3 -1)]$

is a solution to your ODE, then so is:

$y(x) = \arctan[ (5/3) (x^3 -1)]+k$

for some constant $k$.

You have to choose $k$ so that the condition $y(1)=5 \pi$ is satisfied, and for that you need $k=5 \pi$.

So the solution to your ODE and condition is:

$
y(x) = \arctan[(5/3) (x^3 -1)] + 5 \pi
$

RonL
• Jun 4th 2008, 03:44 PM
ArTiCK
Hi all,

could someone post full working out please because i still can't get the answer. When i do it, i get c = tan(5*pi) -5/3

y = arctan( (5/3)x^3 + c)

Thanks,
ArTiCk