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प्रश्न
Choose the correct alternative answer for the following question .
In an A.P. first two terms are –3, 4 then 21st term is ...
विकल्प
-143
143
137
17
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उत्तर
It is given that,
a = –3
a2 = 4
We know that,
\[a_2 = a + \left( 2 - 1 \right)d\]
\[ \Rightarrow 4 = - 3 + d\]
\[ \Rightarrow d = 7\]
Now,
\[a_{21} = a + \left( 21 - 1 \right)d\]
\[ = - 3 + 20\left( 7 \right)\]
\[ = - 3 + 140\]
\[ = 137\]
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