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## #1 2019-01-13 04:45:38

Zeeshan 01
Member
Registered: 2016-07-22
Posts: 746

### Derivative

F(x)=e^x * cos (x-a)
Want to exercise this type of question
Know simple derivation but want to get this answer that is
Derivative=re^x cos(x-a+thetha)
R=sqrt 2 thetha=pi/4

M.M.Zeeshan.K

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## #2 2019-01-13 20:36:56

Bob
Registered: 2010-06-20
Posts: 8,914

### Re: Derivative

hi Zeeshan 01

There are two stages to this problem.  I think you have done the first but I'll say anyway.

This is a product of two functions so use the product rule:

You can combine the trig functions into a single trig function using the compound angle formulas.  You want a cosine so I'll use

To make the derivative look like the right hand side here:

Hope that helps,

Bob

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

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