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Help me to clear my confusion, this is regarding nth derivative of e^ax cos(bx+c) 
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#1
Mar112, 11:48 PM

P: 11

My question is about the nth derivative of e^ax cos(bx+c). Though i can calculate it easily but i am confused at one point.
When we calculate the first derivative we put a = r.cos(theta), b = r.sin(theta) (every thing is ok till here) My confusion starts when we use (theta) = tan^1(b/a) [tan inverse] the reason for my confusion can be understood by: suppose we have a = 1, b = 1 we put a = sqrt{2}*cos(3*pi/4) b = sqrt{2}*sin(3*pi/4) but the tan^1(b/a) = tan^1(1) = pi/4 but our theta is 3*pi/4 according to this theta our a will be 1 and b will be 1 which is different from our values of a and b 


#2
Mar212, 03:53 PM

Sci Advisor
P: 6,106

tan is periodic with period π, so arctan (1) = π/4 + kπ, for any integer k.



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