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प्रश्न
If 7th and 13th terms of an A.P. be 34 and 64 respectively, then its 18th term is
विकल्प
87
88
89
90
उत्तर
89
\[a_7 = 34\]
\[ \Rightarrow a + 6d = 34 . . . . . \left( 1 \right)\]
\[\text { Also,} a_{13} = 64\]
\[ \Rightarrow a + 12d = 64 . . . . . \left( 2 \right)\]
Solving equations
\[\left( 1 \right) \text { and } \left( 2 \right)\], we get:
a = 4 and d = 5
\[\therefore a_{18} = a + 17d\]
\[ = 4 + 17\left( 5 \right)\]
\[ = 89\]
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