The combination is .
The combinations formula is .
(Apply combinations formula)
()
(Expand )
.
The total number of combinations are 4.
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.![\"\"](\"/test/fckeditor/editor/plugins/fckeditor_wiris/integration/showimage.php?formula=200b090192af00b05d6e60bb5f33432a.png\")
.![\"\"](\"/test/fckeditor/editor/plugins/fckeditor_wiris/integration/showimage.php?formula=448970f2cecab66d958a158ebd75e780.png\")
(Apply combinations formula)![\"\"](\"/test/fckeditor/editor/plugins/fckeditor_wiris/integration/showimage.php?formula=a1b393b9a8fa1ab247a904fa8e97c5f6.png\")
(![\"\"](\"/test/fckeditor/editor/plugins/fckeditor_wiris/integration/showimage.php?formula=aec422402cf4696d04012c8fee6e408c.png\")
)![\"\"](\"/test/fckeditor/editor/plugins/fckeditor_wiris/integration/showimage.php?formula=44487639e4bcdea71387a4cb5ee5294a.png\")
(Expand ![\"\"](\"/test/fckeditor/editor/plugins/fckeditor_wiris/integration/showimage.php?formula=9d399dbc1d01a9f6e20925aed2c5b013.png\")
)![\"\"](\"/test/fckeditor/editor/plugins/fckeditor_wiris/integration/showimage.php?formula=780a6d6f8a36e40bb0bb4dfeb69b1251.png\")
. ![\"\"](\"http://www.mathskey.com/userfiles/image/steps_images/solution.JPG\")