百度作业网GMAT基础数学题,希望牛人看看
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GMAT基础数学题,希望牛人看看
Tofurnish a room in a model home,an interior decorator is to select 2 chairs and2 tables from a collection of chairs and tables in a warehouse that are alldifferent from each other.If there are5 chairs in the warehouse and if 150 different combinations are possible,howmany tables are in the warehouse?
(A) 6
(B) 8
(C) 10
(D) 15
(E) 30
我把方程列出来了,可是不会解方程,有牛人点拨下吗,
/2!(X-2)!= 15
/(X-2)!= 30
X(X-1)(X-2)!/ (X-2)!= 30
X(X-1) = 30
X2 – X – 30 = 0
X2 – 6X + 5X – 30 = 0
X(X – 6) + 5(X – 6) = 0
(X – 6) (X + 5) = 0
X = 6 or X = -5
Negative value is not possible since we are calculating the number of tables
Therefore X = 6
Hence “A” is the answer
X(X-1)(X-2)!/ (X-2)!= 30 是怎么从X!/(X-2)!= 30 变过来的哦?
怎么就变成了x(x-1)了?
Tofurnish a room in a model home,an interior decorator is to select 2 chairs and2 tables from a collection of chairs and tables in a warehouse that are alldifferent from each other.If there are5 chairs in the warehouse and if 150 different combinations are possible,howmany tables are in the warehouse?
(A) 6
(B) 8
(C) 10
(D) 15
(E) 30
我把方程列出来了,可是不会解方程,有牛人点拨下吗,
/2!(X-2)!= 15
/(X-2)!= 30
X(X-1)(X-2)!/ (X-2)!= 30
X(X-1) = 30
X2 – X – 30 = 0
X2 – 6X + 5X – 30 = 0
X(X – 6) + 5(X – 6) = 0
(X – 6) (X + 5) = 0
X = 6 or X = -5
Negative value is not possible since we are calculating the number of tables
Therefore X = 6
Hence “A” is the answer
X(X-1)(X-2)!/ (X-2)!= 30 是怎么从X!/(X-2)!= 30 变过来的哦?
怎么就变成了x(x-1)了?
X(X-1)【(X-2)!】 / (X-2)! = 30
这样更规范
约分后 就剩下x(x-1)=60了
再问： 不对呀，是X(X-1)=30 我不明白X!怎么就变成了x(x-1). 请教~
再答： 打错 是30 O(∩_∩)O~ 只能说 X!变成了X(X-1)【(X-2)!】 即X!=X(X-1)【(X-2)!】 比如5！=5乘以4乘以3！ 一样
这样更规范
约分后 就剩下x(x-1)=60了
再问： 不对呀，是X(X-1)=30 我不明白X!怎么就变成了x(x-1). 请教~
再答： 打错 是30 O(∩_∩)O~ 只能说 X!变成了X(X-1)【(X-2)!】 即X!=X(X-1)【(X-2)!】 比如5！=5乘以4乘以3！ 一样