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anygoal [31]
1 year ago
13

What is a21 of the arithmetic sequence for which a7 = -19 and a10 =-28

Mathematics
1 answer:
AlekseyPX1 year ago
3 0

a_{n} = a_{1} + (n - 1)d \\ a_{7} = a_{1} + (7 - 1)d \\ a_{7} = a_{1} + 6d \\ a_{10} = a_{1} + 9d

- 19 = a_{1}  + 6d \\  - 28 =  a_{1} + 9d \\  \\  - 19 =  a_{1} + 6d \\ 28 =  -  a_{1} - 9d \\\  - 3d = 9 \\ d =  - 3

a_{1} =  - 19 - 6d =  - 19 - 6( - 3) =  - 1 \\

a_{21} =  a_{1} + (21 - 1)d \\ a_{21} =  - 1 + 20( - 3) =  - 61

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