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**Derivative**

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

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

Malik

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**Bob****Administrator**- Registered: 2010-06-20
- Posts: 9,262

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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**Derivative**