The number corresponding to 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 is equal to 10, for example: 1 + 9

The number corresponding to 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 is equal to 10, for example: 1 + 9

=(1+9)x9/2
=10x4.5
=45