Question 730193
t5 = a*r^4=15

t8 = ar^7= -15/8



{{{t8/t5 = ar^7/ar^4 = -(15/8)/15}}}

{{{r^3=-1/8}}}

r=-1/2

ar^4 = 15

a*(-1/2)^4=16

a*(1/16)=15
a= 240

T3 = 240*(1/4)
T3= 60

CHECK

t8= 240 * (-1/2)^7

= 240*(-1/128)

=-15/8