Question

a- 4000
d: 400
n=?
tn=8000

Answer 1

a+(n-1)d=0

given

a=4000 d=400

put formula

4000+(n-1)400=0

(n-1)400=-4000

(n-1)=-4000/400

(n-1)=10

n=10+1

n=11

Answer 2

Answer:

tn = [a+(n-1)d]

8000 = [4000+(n-1)(400)

4000= 400n-400

400n = 3600

n = 9

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