Choose three different numbers from 1,2,3... 20, and make these three numbers into arithmetic sequence. How many pairs of arithmetic sequence are there?

The difference is + 1
It can start with any number from 1 to 18
The difference is + 2
It can start with any number from 1 to 16
The difference is + 3
It can start with any number from 1 to 14
The difference is + 4
It can start with any number from 1 to 12
.
The difference is + 9
It can start with any number from 1 to 2
2+4+6+.+18=90
90 pairs can be selected
But the sequence can also be decreasing, that is, in reverse order
So a total of 90 × 2 = 180 equal difference series can be formed

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