Since the number of seats for each subsequent row is increasing by 4, we have an arithmetic progression, with the first term equal 18 and the ratio equal 4.
In order to find the number of seats, knowing that the amphitheatre has 30 rows, we can use the formulas:
[tex]\begin{gathered} a_n=a_1+(n-1)\cdot r \\ S=\frac{n(a_1+a_n)}{2} \end{gathered}[/tex]Using n = 30, a1 = 18 and r = 4, we have:
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