\
The expression is
.
\
Apply sine sum identity :
.
\
\
\
.
![\"\"](\"http://www.mathskey.com/userfiles/image/steps_images/step1.JPG\")
![\"\"](\"/test/fckeditor/editor/plugins/fckeditor_wiris/integration/showimage.php?formula=f04988623de4718116ad74dbbbe3ff24.png\")
.![\"\"](\"/test/fckeditor/editor/plugins/fckeditor_wiris/integration/showimage.php?formula=c260065372eac3e4c6637ce8f16f61fd.png\")
.![\"\"](\"/test/fckeditor/editor/plugins/fckeditor_wiris/integration/showimage.php?formula=13240f19558e733d27905cbcf790f15e.png\")
![\"\"](\"http://www.mathskey.com/userfiles/image/steps_images/solution.JPG\")
![\"\"](\"/test/fckeditor/editor/plugins/fckeditor_wiris/integration/showimage.php?formula=eb08f69ccd203a2c9d4e6c5a40389779.png\")
.