# stuck in a simple step after differentiation.

• March 8th 2010, 04:41 AM
avengerevenge
stuck in a simple step after differentiation.
pls help me with this simple step
i dont know how that last step came,
• March 8th 2010, 05:50 AM
Jester
Quote:

Originally Posted by avengerevenge
pls help me with this simple step
i dont know how that last step came,

From what you have

$
y - a \sin ^3 t = - \tan t \left(x - a \cos ^3 t\right)
$

$
y \cos t - a \sin^3t \cos t = - x \sin t + a \cos^3t \sin t
$

$
y \cos t = - x \sin t +a \sin^3t \cos t + a \cos^3t \sin t
$

$
y \cos t = - x \sin t +a \sin t \cos t\left( \underbrace{\cos^2t + \sin^2t}_{=1}\right)
$

so

$
y \cos t = - x \sin t +a \sin t \cos t$
.
• March 8th 2010, 06:08 AM
avengerevenge
thanks
i know this is very simple step but it was not coming,
thankss