arithmetic
posted by charles .
first term and larst term of a geometric progression is 42.if the forth term is greater than the second term by 168, find the first term.the forth term.

check your typing.
If the first and last term of a GS are the same, then r = 1, that is, the terms do not change.
But then how can there be a difference between the fourth and second term?
Did you mean..
"The sum of the 1st and last terms is 42" ?
Either way, you would have a major problem:
you have 3 unknowns, a, r, and n
but only two sets of information.