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Find n and r if. (a)^nPr = 720 and ^nC(n...

Find n and r if. `(a)^nP_r = 720 and ^nC_(n-r) = 120` (b) `^nC_(r-1) : ^nC_r : ^nC_(r+1) = 20 : 35 : 42`

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The correct Answer is:
a) n=10, r=3, b) n=10, r=4
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