# Prove the following identities

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• Apr 25th 2013, 06:02 PM
choi105
Prove the following identities
I'm not sure the working step.

tan x + tan y = sin(x + y) / cos x + tan y

This my ans;

LHS = sin x cos y + cos x sin y / cos x cos y
= sin x + sin y
= tan x + tan y
= RHS
• Apr 25th 2013, 06:57 PM
chiro
Re: Prove the following identities
Hey choi105.

I think you may have copied this down wrong.

Do you mean sin(x+y)/[cos(x) + tan(y)] instead?
• Apr 25th 2013, 07:05 PM
choi105
Re: Prove the following identities
yup.
• Apr 25th 2013, 10:29 PM
ibdutt
Re: Prove the following identities
The identity written is not true. Please recheck the question.
• Apr 26th 2013, 07:17 PM
choi105
Re: Prove the following identities
sorry for typing error.
The que: tan x + tan y = sin(x + y) / cos x + cos y
• Apr 26th 2013, 08:30 PM
sa-ri-ga-ma
Re: Prove the following identities
Again it is wrong. Check the Dinominator.
• Apr 26th 2013, 11:06 PM
ibdutt
Re: Prove the following identities
I think RHS would be having cosx cos y in the denominator.
write tanx and tany in the form of sinx / cosx and siny/cosy. take LCM and use the formula for sum of angles the numerator and you have the result.