Question

19) Which term of the A.P -3,-5,-7,-9 _ _ _ _ is -21​

Answer 1

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AP = -3,-5,-7. . .

a1=-3 , a2=-5

d = a2-a1

d = -2 and An = -21

Now,

we know that,

• An = a+(n-1) d

$- 21 = - 3 + (n - 1) \times - 2 \\ - 21 + 3 = - 2n + 2 \\ - 18 = - 2n + 2 \\ - 2n = - 20 \\ n = \frac{20}{2} \\ n = 10$

So, the 10th term of an AP is -21

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