if the 7th term of an a.p is twice the third term and the sum of the first four terms is-16 and the last and the last term is 72, find the number of the terms

Answer 1

show me the answer

Answer 2

Let a_n be the n^th term of arithmetic progression a. Can you then start to write out your ap using symbols?

Answer 3

not so.
I'll show you how to find the answer.
Using the formula for the nth term of an AP, you know that
a+6d = 2(a+2d)
4/2 (2a+3d) = 16
Now you can find a and d, so you just need to know how many terms there are.
If there are k terms, then
a+(k-1)d = 72
so using a and d found in part 1, solve for k.