having a problem in trigonometry

Hi Abdellah Elayou,

I see from your question that you already see that it isn't as easy as with sin(x) or cos(x). I think you mean the sign (+ or -) in front of the sin or cos? The minus sign in front of sinus or cosinus flips the whole function round the x-axis. So if you know how the function looks like without a minus sign, you look whether it is mirrored. So with advanced trigonometric functions you can't tell it on the first sight.

But do you already know that you can transform sinus in cosinus and the other way round? So a sinus is the same as a cosinus moved a half pi to the right: g(x) = cos(x-(1/2*pi)). So if a graph is given there are multiple possible functions for it.

I can not recomment you a video, although I think I will repeat some of this stuff soon for my last exam, I think the videos here at Khan Academy are all good.

But I would recommend you to download GeoGebra, a free program to draw graphs and many more. Just try out different parameters and signs, this is the way of learning Analysis I would recomment, to get a feeling for the functions.

See you soon with new questions, --Okapi