Question

which term of the AP 24,21,18,15,... is the first negative term

Answer 1

-3 is the answer of this

Answer 2

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$here \: a = 24 \: and \: d = (21 - 24) = - 3 \\ let \: the \: nth \: term \: of \: the \: given \: ap \: be \: the \: first \: negative \: term \\ then \: t _{n} < 0 \implies \: a + (n - 1)d < 0 \\ \implies \: 24 + (n - 1) \times - 3 < 0 \\ \implies(27 - 3n) < 0 \implies27 < 3n \\ \implies3n > 27 \\ \implies \: n > 9 \\ \therefore \: n = 10 \\ hence \: first \: negative \: term \:is \: 10$