# Math Help - Does this Look right?

1. ## Does this Look right?

Suppose
and is negative. Here are some small variations on the previous problems:

Which one of the two addition formals would put me on the right way?
sin(u-pi)=sin(u)cos(pi)-cos(u)sin(pi)
or
sin(-(12/13)-pi)= sin-(12/13)cos(pi)-cos-(12/13)sin(pi)?

2. ## Re: Does this Look right?

I would use:

$\sin(u)=-\sqrt{1-\cos^2(u)}$

$\sin(u-\pi)=-\sin(\pi-u)=-\sin(u)$

$\cos(u-\pi)=\cos(\pi-u)=-\cos(u)$

$\sin\left(u-\frac{\pi}{2} \right)=-\sin\left(\frac{\pi}{2}-u \right)=-\cos(u)$

$\cos\left(u-\frac{\pi}{2} \right)=\cos\left(\frac{\pi}{2}-u \right)=\sin(u)$

3. ## Re: Does this Look right?

Originally Posted by MarkFL2
I would use:

$\sin(u)=-\sqrt{1-\cos^2(u)}$

$\sin(u-\pi)=-\sin(\pi-u)=-\sin(u)$

$\cos(u-\pi)=\cos(\pi-u)=-\cos(u)$

$\sin\left(u-\frac{\pi}{2} \right)=-\sin\left(\frac{\pi}{2}-u \right)=-\cos(u)$

$\cos\left(u-\frac{\pi}{2} \right)=\cos\left(\frac{\pi}{2}-u \right)=\sin(u)$
Thank You very much, So the initial cos (u)=5/13 has nothing to do with the other problems? I thought I had to relate each one to the original somehow

4. ## Re: Does this Look right?

But the other problem is when I input your answers the program tells me variable "u" is not defined in this context?
meaning these answers it will consider wrong

5. ## Re: Does this Look right?

You are given the value of cos(u), then from this you may find sin(u) using the first formula, and since you now have sin(u) and cos(u), the rest come from that.

6. ## Re: Does this Look right?

but sin(u) is -(12/13)

7. ## Re: Does this Look right?

Which would mean sin(u-pi) should be Sin(u-pi)= sin-(12/13)cos(pi)-cos-(12/13)(sin(pi)

8. ## Re: Does this Look right?

If sin(u) = -12/13, then -sin(u) = 12/13.