The 5th, 9th and 16th term of an AP are consecutive term of a GP. The sum of the 12th term of an AP is 492. Find the first term of AP and the common ratio of the GP
(a+8d)/(a+4d) = (a+15d)/(a+8d)
12/2 (2a+11d) = 492
a=8, d=6
r = 7/4
or, more trivially,
a=41, d=0, r=1