I've seen an exercise on the sequence and I really couldn't solve it! … it was like a skydiver jumped from a height of 900 m, in the first second falls 5 m in the second second falls 16 m, in the second second falls 27 m in the fourth second fell 38m … in 355m he opened his parachute. so the sequence is 5,16,27,38, ….. 355, …. 900, the question is in which second did he open his parachute? (in 355m) … so I got that U1 = 5 & d = 11, but I can't climb to 355m, what second was that? … U32 = u1 + (n-1) * d = 5 + (32-1) * 11 = 346 && U33 = u1 + (n-1) * d = 5 + (33-1) * 11 = 357 Can anyone help ?! … thanks!