唔識Factorial

puuww

Solve the following equation, where n is a positive number.

nC30=nC20

唔係好識點solve
有冇人可以教下我？

sapphire

這個應該不用solve吧

nCr = nC(n - r)

r = 30
n - 30 = 20
n = 50

puuww

這個應該不用solve吧

nCr = nC(n - r)

r = 30
n - 30 = 20
n = 50
sapphire 發表於 23/9/2009 10:45 PM

                               
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照你咁講
如果nC(n-30)=nC20
我地係唔係可以話n-30=20
姐係nCr=nCr 佢地兩個的r一樣？
但係條題目一開始一個係30 一個係20喎

sapphire

我沒提過兩個r的數值是一樣的啊

如果你是說 nCr = nCr的話, 那兩個r當然會一樣
如果是 nCr = nCx, 而 r=/=x 的話, 那就是 n - r = x 了

把上面的寫好一點的話......
By nCr = nC(n - r),
Let r = 30,
n - r = n - 30
By the question, R.H.S. = nC20
so, n - 30 = 20