You MUST know the definitions.
Seventh term of geometric series is 64
---> ar^6 = 64 **
sum of its 10 terms is 1023
---> a(r^10 - 1)/(r-1) = 1023 ***
divide *** by ** , the a will cancel
(r^10 - 1)/(r^6(r-1)) = 1023/64
64r^10 - 64 = 1023r^7 - 1023r^6 = 1023r^6(r-1)
took a "wild guess" at r = 2 , by knowing that 2^10 = 1024
LS = 64(1024)-64 = 65472
RS = 1023(64) = 65472
if r = 2, the a = 1 form ***
term15 = ar^14
= 16384
Seventh term of geometric series is 64 and sum of its 10 terms is 1023. Find its 15th term.
1 answer