OpenStudy (anonymous):

solve cos (7x)=0 on the interval [0,2pi/7]

OpenStudy (anonymous):

Let's replace $$7x$$ with another variable -- how about $$u$$? Can you solve $$\cos u=0$$ on the interval $$[0,2\pi]$$?

OpenStudy (anonymous):

Phew thanks for the reply...So cosx=0 at 0 and 2pi right??

OpenStudy (anonymous):

No... try sketching a unit circle.

OpenStudy (anonymous):

oh its at pi/2, 3pi/2...

OpenStudy (anonymous):

@tanentrahan indeed. Now convert back to $$x$$. Remember, $$u=7x$$ so $$x=\frac17u$$. Since we solved for $$u$$ in $$[0,2\pi)$$ we will get $$x$$ in $$\left[\frac07,\frac{2\pi}7\right)=\left[0,\frac{2\pi}7\right)$$.

OpenStudy (anonymous):

Thanks!!! I'm not sure if i completely understand what happened though. We isolated the "x" and solved for it right??

OpenStudy (anonymous):

So for this one cos(7x)=-1 the interval being 0, pi/2 our x value is -1/7 right?