Question

9. If the 3rd and the 9th terms of an AP are 4 and - 8 respectively, which term of this AP is zero?​

Answer 1

Answer:

let the 1st term is a and common difference is b

so

a+(3-1)b=4

a+2b=4, a=4-2b

again,

a+(9-1)b= -8

a+8b= -8, a= -8-8b

4-2b= -8-8b

-2b+8b= -8-4

6b= -12

b= -12/6= -2

a=4-2b=4-2.(-2)=4+4=8

let the nth term is 0

a+(n-1)b=0

8+(n-1)(-2)=0

8-2n+2=0

2n=10

n=10/2=5

5th term of this AP is zero

Answer 2

Answer:

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Answer is in the attachment.

Answer is 11 years.

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