Question

An arithmetic progression has the same first and second terms as the geometric progression.

(ii) Find the sum of the first 20 terms of the arithmetic progression.

Answer 1

AP: 1,11,21,31,...

GP: 1,11,121,1331,...

Explanation:

first term: a

2nd term a+d and ar

so, a+d = ar

r = 1 + d/a

So, pick an a&d, say 1&2.

AP: 1,3,5,7,9...

GP: 1,3,9,27,81,...

or, 10&5:

AP: 1,11,21,31,...

GP: 1,11,121,1331,...

If no other information is given, that's about as far as we can go.