# Math Help - differentiating parametric equations

1. ## differentiating parametric equations

x=(4+sin 20 t) cos t
y=(4+sin 20 t) sin t
z= cos 20 t

not sure what rule to use to get
dx/dt?
dy/dt?
dz/dt?

2. Originally Posted by harveyo
x=(4+sin 20 t) cos t
y=(4+sin 20 t) sin t
z= cos 20 t

not sure what rule to use to get
dx/dt?
dy/dt?
dz/dt?
use product and chain rule,..

for the first one,.

$x=(4 + \sin 20 t) \cos t = 4\cos t + \sin 20t \cos t$

do the rest

$\frac{dx}{dt} = -4 \sin t + ( 20 \cos 20t \cos t - \sin 20t \sin t )$

3. Frankly, by the time a person is studing three dimensional problems and parametric equations, differentiating trig functions should be easy. What was giving you difficulty here? And why in the world is this posted under "Differential Equations"?

By the way, I would not multiply the first one:
f(x)= (4+ sin(20t))cos(t) so f'= (4+ sin(20t))'cos(t)+ (4+ sin(20t)(cos(t))'
= (20cos(20t))cos(t)- (4+ sin(20t)sin(t).