on this sequence the 1st term is 83 or A1=83. the last term is 5. An=5. The common difference is -3.
using the formula in finding An you'll have
An = A1 + (n-1) d
5 = 83 + (n-1)(-3)
5 = 83 -3n +3
transpose -3n to the left and 5 to the right
3n = 83 + 3 + 5
3n = 91
n = 91/3
recheck the given problem
