function Q: tangent of f parallel to tangent of g

[jst706]

Let f and g be the functions defined by f(x) = sin x and g(x) = cos x. For which of the following values of a is the tangent line to f at x = a parallel to the tangent line to g at x = a?

The answer is 3pi/4 but I dont understand how to get to that. I take the derivative of each and set them equal; I get:

cosx = -sinx

But how do I end up in terms of pi? Would I convert it to tan x = -1? If so, how do I get it in terms of pi? Could someone please explain? Thank you!

 

[pka]

Re: function question

cosx=-sinx but how do I end up in terms of pi? would I convert it to tanx=-1? if so, how do I get it in terms of pi? could someone please explain.

You are just expected to know that bit of information!
It is in any reasonably good trigonometry textbook.
That is just general information that anyone must know to pass a course in trigonometry.