Question

The distance in feet that a free-falling body falls in each second, starting with the first second, is given by the sequence 21, 63, 105, 147, . . . . Find the distance that the body falls in the 7th second.

Answer 1

it is an arithmatic progression
a= 21
d= 42
so a5=a+(4)*d
=21 + 168
= 189