How many bit strings are there of length 9 with exactly two 1s? How many 2-element subsets are there of a set with 9 elements?

👨🏻‍🏫 學術台數學

[匿名]

2016-06-23 6:43 am

回答 (1)

Lopez

2016-06-23 10:37 am

✔ 最佳答案

(1)
9! / ( 2! * 7! )
= 9*8 / (2*1)
= 36 ..... Ans

(2)
C( 9 , 2 )
= 9*8 / (2*1)
= 36 ..... Ans

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