作业帮我做的答案是E.但实际为A,why?我理解的是在r*r+1个方形组成的长方形中,哪一个选项代表的方形既不在R4之前也不在C7之前的区域里?求正解
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我做的答案是E.但实际为A,why?我理解的是在r*r+1个方形组成的长方形中,哪一个选项代表的方形既不在R4之前也不在C7之前的区域里?求正解
我做的答案是E.但实际为A,why?
我理解的是在r*r+1个方形组成的长方形中,哪一个选项代表的方形既不在R4之前也不在C7之前的区域里?
求正解
我做的答案是E.但实际为A,why?我理解的是在r*r+1个方形组成的长方形中,哪一个选项代表的方形既不在R4之前也不在C7之前的区域里?求正解
You can use inclusion and exclusion.
We know that there are a total number of r * (r + 1) = (r^2 + r) squares.
There are r + 1 squares in each row and r squares in each column,which implies that 4th row has r+1 squares and 7th column has r squares.
In order to find the number of squares that are neither in the 4th row nor in the 7th column,you need to subtract r+1 and r from (r^2 + r),which gives r^2 - r -1.
But since you subtract the square at 4th row 7th column twice,you need to add it back once.
Therefore,the result should be r^2 - r -1 + 1 = r^2 - r,which is simply choice A.
Hope that will be helpful!