1. ## [SOLVED] quick differentiation

does dy/dt of 2.5sin((pi)t/12) = 2.5cos((pi)t/12)?

2. Originally Posted by PIR
does dy/dt of 2.5sin((pi)t/12) = 2.5cos((pi)t/12)?
No

The derivative of $2.5 \sin \left(\frac{\pi t}{12}\right)$ is $\frac{2.5\:\pi}{12}\:\cos \left(\frac{\pi t}{12}\right)$

The derivative of f(ax+b) is : a f'(ax+b)

3. Originally Posted by PIR
does dy/dt of 2.5sin((pi)t/12) = 2.5cos((pi)t/12)?
No it doesn't. You need to apply the chain rule on that $\frac{\pi t}{12}$

Remember that by the chain rule if $f(x) = Asin(wt)$ then $f'(x) = Awcos(wt)$

4. Thank you you two. Maths in the morning doesn't work for me I really should have picked up the chain...

5. Originally Posted by PIR
does dy/dt of 2.5sin((pi)t/12) = 2.5cos((pi)t/12)?
Use the chain rule.

$\frac{d}{dt} K \sin( Rt) = KR\cos(Rt)$

Where K and R are constants.