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Prove the equation below is an identity.

(1+sin x)/(1 - sin x) =2sec2x + 2sec x tan x - 1

 

I understand identity when it comes to basic equations but this one just goes past my head.

Thank you for whoever has time for this!

 Nov 12, 2015

Best Answer 

 #1
avatar+130511 
+10

I think it's supposed to be "2sec^2x "  on the right side

 

(1+sin x)/(1 - sin x) =2sec^2x + 2sec x tan x - 1

 

Multiply top/bottom  on the left hand side by ( 1 + sinx)

 

[ 1 + sinx) (1 + sinx)] / [ (1 - sinx) (1 + sin x) ]  = 2sec^2x + 2sec x tan x - 1  

 

Simplify the LHS and remember that [ 1  = sec^2x - tan^2x]      

 

[ 1 + 2sinx + sin^2x] / (cos^2x]  =  2sec^2x  + 2secxtanx - [ sec^2x - tan^2x]

 

[ 1 + 2sinx + sin^2x] / (cos^2x]   = sec^2x + 2secxtanx + tan^2x

 

And, on the left, we can write :

 

1/cos^2x + [2sinx]/ [ cosx * cosx]  + [ sin^2x / cos^2x]

 

sec^2x  + 2[sinx/cosx] * [1/ cosx]  + tan^2x

 

sec^2x  + 2tanxsecx + tan^2x

 

sec^2x + 2secxtanx + tan^2x        and this equals the RHS

 

 

 

cool cool cool

 Nov 12, 2015
 #1
avatar+130511 
+10
Best Answer

I think it's supposed to be "2sec^2x "  on the right side

 

(1+sin x)/(1 - sin x) =2sec^2x + 2sec x tan x - 1

 

Multiply top/bottom  on the left hand side by ( 1 + sinx)

 

[ 1 + sinx) (1 + sinx)] / [ (1 - sinx) (1 + sin x) ]  = 2sec^2x + 2sec x tan x - 1  

 

Simplify the LHS and remember that [ 1  = sec^2x - tan^2x]      

 

[ 1 + 2sinx + sin^2x] / (cos^2x]  =  2sec^2x  + 2secxtanx - [ sec^2x - tan^2x]

 

[ 1 + 2sinx + sin^2x] / (cos^2x]   = sec^2x + 2secxtanx + tan^2x

 

And, on the left, we can write :

 

1/cos^2x + [2sinx]/ [ cosx * cosx]  + [ sin^2x / cos^2x]

 

sec^2x  + 2[sinx/cosx] * [1/ cosx]  + tan^2x

 

sec^2x  + 2tanxsecx + tan^2x

 

sec^2x + 2secxtanx + tan^2x        and this equals the RHS

 

 

 

cool cool cool

CPhill Nov 12, 2015
 #2
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0

Yes! You're right about the problem being wrong! I couldn't figure out how to make something dquared using a keyboard and I didn't remember to state that! Thank you for your help!

 Nov 12, 2015

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