Arithmetic Geometric Sequence

1.
the general term of Geometric Sequence = aRn-1
The 3rd and 5th terms of a geometric sequence are 60 and 15 respectively
aR2=60---------1
aR4=15---------2
1/2:
1/R2=4
R=1/2 or R=-1/2 (rejected) [all the terms are positive]
when R=1/2, a=240
the sum to infinity of the geometric sequence
=a/(1-R)
=240/(1-1/2)
=480
Answer : D
2.
x^2 , -x^2 , -3x^3 , ... is a geometric sequence
-x^2/x^2=-3x^3/-x^2
-1=3x
x=-1/3
Answer : C