Question

the sum of the first six terms of a geometric series is 21/16 . if the common ratio is -1/2 , what is the first term?

Answer 1

Answer:

-84

Step-by-step explanation:

The formula for the nth term of a geometric progression is

Un = ar^n- 1

The sum of the first six terms of a geometric series (U6) = 21/16

Common ratio = -1/2 ,

a = First term ??

21/16 = a × (-1/2 )^6- 1

21/16 = a ×(-2^-1)^5

21/16 = a × -2^-6

Cross Multiply

16 × a × -2^-6 = 21

a = 21/16 × -2^-6

a = 21/16 ×- 1/2⁶

a = 21/16 × -1/64

a = 21 ÷(-1/4)

a = 21 × -4

a = -84

The first term = 84