What am I doing wrong?

  • Thread starter powp
  • Start date
  • #1
powp
91
0
Hello

I have to prove that

(cos 3x - Cos 7x) / (Sin 3x + Sin 7x) = Tan 2x

So this is what I am doing

-2 Sin (3x + 7x)/2 Sin (3x-7x)/2
--------------------------------
2 Sin (3x + 7x)/2 Cos (3x-7x)/2

-2 Sin 5x Sin -2x
-----------------
2 Sin 5x Cos -2x

Sin 2x
-------
-cos 2x

Which equals -Tan 2x. What am i doing wrong??

Thanks
 

Answers and Replies

  • #2
Yegor
147
1
-2 Sin 5x Sin -2x
-----------------
2 Sin 5x Cos -2x

Sin 2x
-------
-cos 2x

Cos(-2x)=Cos(2x)
Then i believe everything is ok.
 
  • #3
powp
91
0
Thanks for your help.
 

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