How many cubes are used to build this
solid?
15 cubes
19 cubes
12 cubes
18 cubes

18 cubes. the first row is 9, the second row is 6, then the third row is 3, add them all up together and creates 18 cubes.

hello from mrbilldoesmath!

answer:

a(n) - a(n-1) -3

discussion:

this ones a bit interesting. given a(n) = 5 - 3n consider this:

a(n-1) = 5 - 3(n-1)

          = (5 - 3n) + 3             => but a(n) = 5 - 3n, the first term

          = a(n)   + 3                 => subtract 3 from both sides

a(n-1) -3 = a(n)  

or

a(n) - a(n-1) -3

you,

mrb