Okay i will give you an example.No I meant how did you get
a+4d/a=a+14d/a+4d
1,2,4,8,
now this is a geometric progression, of ratio 2.
and you see,
2/1 = 4/2 = 8/4
2 = 2 = 2
Always in geometric progression when you divide a number with the one before is equal when you divde a different number with the number before ( hope u get my point. )
Therefore
a, a+4d, a + 14d
a+4d/a=a+14d/a+4d