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  • The 21st term of the AP whose first two terms are –3 and 4 is (A) 17 (B) 137 (C) 143 (D) –143

The 21st term of the AP whose first two terms are –3 and 4 is

(A) 17

(B) 137

(C) 143

(D) –143

Answers (1)

Given

\\a\textsubscript{1} = - 3\\ a\textsubscript{2} = 4\\ $(Common difference) $d = a\textsubscript{2} - a\textsubscript{1}\\ \ = 4 - (-3)\\ \ = 4 + 3 \\ \ = 7 \\$ We know that$\\ \ a\textsubscript{n}\ =\ a\ +\ (n\ -\ 1)d \ \ $put n$ = 21\\ a\textsubscript{21} = a + (21 - 1).d\\ \ \ \ \ = - 3 + (20).(7)\\ \ \ \ \ = - 3 + 140 \\ \ \ \ \ = 137\\

Hence option B is correct.

Posted by

infoexpert21

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