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4. Which term in the arithmetic sequence 11, 4, -3...is - 129?
5. In an arithmetic sequence, if a 8 = 51 and a l3= 91, find al, d and a 25.
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Answer:

-129 is in 21st term

-129 is in 21st terma21 = -129

Step-by-step explanation:

1st step : solve for common difference:

d = a2-a1

d = 4-11

d = -7

2nd step: solve for which term is -129:

an = a1+(n-1)d

-129 = 11+(n-1)-7

-129 = 11 + (-7n) + 7

-129 = 11 -7n +7 {combine similar term}

-129 = 18 - 7n

-129 - 18 = -7n

-147 = -7n {divide both side by -7}

21-n

n = 21

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