# Chairs

Determine the number of seats in the seventh row and ninth row, if 3rd row has 14 seats and in every next row of seats has five more than the previous row.

Correct result:

a7 =  34
a9 =  44

#### Solution:

$a_{3}=14 \ \\ d=5 \ \\ a_{3}=a_{1}+2d \ \\ a_{1}=a_{3}-2 \cdot \ d=14-2 \cdot \ 5=4 \ \\ a_{7}=a_{1}+6 \cdot \ d=4+6 \cdot \ 5=34$
$a_{9}=a_{1}+8 \cdot \ d=4+8 \cdot \ 5=44$

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