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Answer:
an = 896 - 53(n - 1)
[tex]a _{19} = l + (19 - 1) \times d \\ - 58= l+ 18d - - - (i) \\ \\ a _{21} = l + (21 - 1)d \\ - 164 = l + 20d - - - (ii) \\ (ii) - (i) : \: - 106 = 2d \\ d = - 53 \\ hence : l = - 58 - (18 \times - 53) \\ l = 896 \\ = > a _{n} = 896 + (n - 1) \times - 53 \\ a _{n} = 896 - 53 (n - 1)[/tex]