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# Differentiating Trignometic Functions - Help? watch

1. I have to differentiate:

cos(½π-5x)

My answer comes out to be:

-5sin(½π-5x)

Answer in the back of the book is:

5cos5x - which one is right?

2. (Original post by entiti)
I have to differentiate:

cos(½π-5x)

My answer comes out to be:

-5sin(½π-5x)

Answer in the back of the book is:

5cos5x - which one is right?

Back of the book. Yours would be right if you'd included another minus.

cos(90 - x) = sin x.
3. ok, thanks for your help.

That's because ½π = 90 ( in degrees ). cos( 90 - x ) = sin x is a basic rule right? Correct me if I'm wrong.
4. yes, so:

y = cos(pi/2 - 5x)
y = sin 5x
dy/dx = 5cos 5x

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