shorter side : longer side = 3 : 7
Equate 3 parts tp 9 inches:
3 parts = 9 inches
Find 1 part:
1 part = 9 ÷ 3 = 3 inches
Find 7 parts:
7 parts = 3 x 7 = 21 inches
Area = Length x Width = 21 x 9 = 189 in²
Answer: The longer side is 21in and the area is 189 in²
The area of the scale drawing would be 1080 square centimeters or (1080 cm^2). I got this answer through first finding the area of the classroom in feet (which is A= lw = [18][20] = 360 ft^2) and then I proceeded to converting it to centimeters via given scale factor (3 cm = 1 foot) and had the answer of 1080 centimeters squared.
I'm guessing the series is supposed to be

By the ratio test, the series converges if the following limit is less than 1.

The first

terms in the numerator's denominator cancel with the denominator's denominator:


also cancels out and the remaining factor of

can be pulled out of the limit (as it doesn't depend on

).

which means the series converges everywhere (independently of

), and so the radius of convergence is infinite.