Hello people,

forgive me for my dumb questions,

but I am asking this question, which might seem stupid for some, in order to avoid paying almost 66 euros for a Wolfram Alpha Pro account (and risking not understanding it anyways)

Reading a book I got stuck in this (uploaded an image under the following link)

x(t) = A*cosine(wt+Phi)

The first derivative is:

dx/dt = -Aw*sin(wt+Phi)

I know that this result is right, for the first derivative,

i just dont know how to get there, (step by step)

Both wolfram alpha

http://www.wolframalpha.com/input/?i=di ... ation+to+t

and Symbo lab

https://www.symbolab.com/solver/derivat ... )%5Cright)

give me the same results,

but Matlab doesn't explain it step by step

Symbolab in other hand explains kind of

but i still didnt understand why

I know what is the chain rule,

and i know that i should differentiate what is in side the parenthesis (wt+Phi) (which by the way is = w)

and I also understand that d/du cos(u) = -sin(u)

-A*sin(wt+Phi)*w

but i sincerely cannot get so far, the reason of thiw last "w"

in my limited comprehension the result should be = -A*sin(wt+Phi)

can anyone save me?

Thanks

Hans