First slide

Arithmetic progression

Question

If 7th term of an A.P. is 9 and 9th term of the A.P. is 7, then 20th term of the A.P. is

Moderate

A

-2
B

-3
C

-4
D

-6

Solution

If d is common difference of the A.P., then
$\begin{array}{l} 2 d = a_{9} - a_{7} = 7 - 9 = - 2 \\ \Rightarrow d = - 1 . \\ ∴ a_{20} = a_{9} + (20 - 9) d = 7 - 11 = - 4 \end{array}$

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