Question

in a geometric sequence a6= 1215 and a9 = 32805. Find a1 for this sequence

Answer 1

A6 = 1215
A9 = 32805

A6 = a1 + 5d
A9 = a1 + 8d

a1 +5d = 1215
a1 = 1215 - 5d...........(¡)

a1 + 8d = 32805
a1 = 32805 - 8d.......(¡¡)

from i and ii......
32805-8d = 1215-5d
32805 = 1215 + 3d
3d = 32805- 1215
3d = 31590
d = 31590/3
d = 10530

Now substituting the value of d in either equation i or ii,
a1 + 5d = 1215
a1 = 1215 - 5(10530)
= 1215 - 52650
= -51435(ans)