All the points are (1/2, -9/2)`( 1, -4)`(3/2, -7/2)`(2, -3)`(5/2, -5/2)`(3, -2) `.

There are four pairs (1/2, -9/2)`(9/2,-1/2).

( 1,-4)(4,- 1) (3/2,-7/2)(7/2,-3/2)

(2,-3)(3,-2)

Next, two points are randomly selected from these ten points and will not be repeated.

There is a method in high school: C 10 takes 2=45.

Junior high school knows nothing. You can arrange it yourself as follows:

Let ten be ABCDEFGHIJ to avoid repetition.

A paired with BCDEFGHIJ

B paired with CDEFGHIJ

C paired with DEFGHIJ

D paired with EFGHIJ

E paired with FGHIJ

F paired with GHIJ

G paired with HIJ

H and IJ partner

J and I are partners.

The total is 9+8+7+6+5+4+3+2+ 1=45.

Understand?