Solve the following quiz (2=6, 3=12, 4=20 ....9=?)

Question

Answer 1

2*3=6
3*4=12
similarly
9*10=90

Comments

this can be true only if you add numbers to the puzzle.
no adding nothing you can solve it
9+9=18+42=60
6+6=12+30=40

Answer 2

The answer is 90.

And i used to solve this is a*(a+1)