10. Find a formula for T(n), the nth triangular number (starting with n = 1).

1) on the number line, -1.5 can be seen as going backwards 1.5.
there are -4 groups of -1.5
this means that you have to move in the opposite direction that you would normally move 1.5
4 groups of 1.5 is 6 (because you have to move in the positive direction)
6 is the answer

2) 975 feet below the surface