document.write( "Question 1111681: An unbiased coin is tossed 13 times. In how many ways can the coin land tails either exactly 9 times or exactly 2 times? \n" ); document.write( "
Algebra.Com's Answer #726697 by math_helper(2461)\"\" \"About 
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The 13 tosses can be thought of as 13 places to hold results. In one case we want to distribute two identical items in those 13 places (13C2) and in the other, 9 identical items are distributed amongst the 13 spaces (13C9). \r
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\n" ); document.write( "Dear student,\r
\n" ); document.write( "\n" ); document.write( "\"+nCr+=+n%21%2F%28%28n-r%29%21r%21%29+\"\r
\n" ); document.write( "\n" ); document.write( "\"+13C2+=+13%21%2F%28%2811%21%29%282%21%29%29+=++13%2A12%2F2+=+78+\" <<< the 11! cancels all but 13*12 in the numerator\r
\n" ); document.write( "\n" ); document.write( "\"+13C9+=+13%21%2F%28%284%21%29%289%21%29%29+=+715+\" <<< here, 9! cancels most of the numerator\r
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