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发帖数: 1320 | 1 WHAT IS THE COEFFICIENT OF X2+Y2=Z2 IN THE EXPANSION (X+Y+Z)6? | w********9
发帖数: 8613 | 2 X^2y^2Z^2?
Consider 6 identical boxes each with 3 distinct items, x, y, and z.
First, pick 2 x's from all the boxes. There are 6*5/2 ways. (/2, for the
picking order doesn't matter.)
Next, pick 2 y's from all the unpicked boxes. 4*3/2 ways.
Finally, pick 2 z's from all the unpicked boxes. 1 way.
90 ways.
The answer is 90. |
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