BTGModeratorVI wrote: ↑Sun Jul 19, 2020 1:38 pm
Rectangle ABCD is constructed in the coordinate plane parallel to the x- and y-axes. If the x- and y-coordinates of each of the points are integers which satisfy 3 ≤ x ≤ 11 and -5 ≤ y ≤ 5, how many possible ways are there to construct rectangle ABCD?
396
1260
1980
7920
15840
Answer:
C
Source: Grockit
First notice that, to construct this rectangle, the vertices will share several points.
For example, if the 4 vertices are at (2, 5), (2, -3), (9, 5) and (9, -3), then we get a rectangle.
Notice that there are only 2 different x-coordinates (2 and 9) and only 2 different y-coordinates (-3 and 5)
So, to create the desired rectangle,
we need only choose 2 different x-coordinates and 2 different y-coordinates
So, let's take the task of creating rectangles and break it into STAGES
STAGE 1: Select the 2 x-coordinates
We can choose 2 values from the set {3, 4, 5, 6, 7, 8, 9, 10, and 11}
In other words, we must choose 2 of the 9 values in the set
Since the order in which we choose the numbers does not matter, we can use COMBINATIONS
We can select 2 number from 9 numbers in 9C2 ways (=
36 ways)
STAGE 2: Select the 2 y-coordinates
We can choose 2 values from the set {-5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5}
In other words, we must choose 2 of the 11 values in the set
We can select 2 number from 11 numbers in 11C2 ways (=
55 ways)
By the Fundamental Counting Principle (FCP), we can complete the 2 stages (and thus create a rectangle) in
(36)(55) ways (= 1980 ways)
Answer: C
