We have an arithmetic progression the squares of whose 12h, 13th, and 15th terms form a geometric progression. Find all the ratios of this progression.

Assuming

u_n+1 = u_n + d -or- u_n+1 = u_1 + d*n

for the arithmetic progression, and

u_n+1 = u_n * q -or- u_n+1 = u_1 * q^(n-1)

for the geometric one, how do we go about this? (My results don't match with the answer in the book I took this from.)

Please only hint, don't solve!

Thanks
-- KE