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Area of a circle is in direct variation with the square of the radius of the circle. If the area of a circle is 616 sq.cm., then the radius is 14 cm. Find the area of the circle whose radius is 42 cm.

Options

Option A is correct

5544 sq.cm.

Option B

3850 sq.cm.

Option C

2464 sq.cm.

Option D

4455 sq.cm.

Explanation

The area of a circle AA is in direct variation with the square of its radius rr, which means A=kr2A = k r^2, where kk is the constant of proportionality (in this case, k=πk=\pi). Given: A1=616A_1 = 616 sq.cm. when r1=14r_1 = 14 cm. We can find the constant kk (which is π\pi): 616=k(14)2616 = k (14)^2 616=k(196)616 = k (196) k=616196=227k = \frac{616}{196} = \frac{22}{7}. So, k=πk=\pi. Now, we need to find the area A2A_2 when r2=42r_2 = 42 cm. A2=kr22A_2 = k r_2^2 A2=227(42)2A_2 = \frac{22}{7} (42)^2 A2=227(42×42)A_2 = \frac{22}{7} (42 \times 42) A2=22×(427)×42A_2 = 22 \times (\frac{42}{7}) \times 42 A2=22×6×42A_2 = 22 \times 6 \times 42 A2=132×42A_2 = 132 \times 42 A2=5544A_2 = 5544 sq.cm. Alternatively, since A=kr2A = k r^2, we have A1r12=A2r22\frac{A_1}{r_1^2} = \frac{A_2}{r_2^2}. We notice that r2=42=3×14=3r1r_2 = 42 = 3 \times 14 = 3 r_1. So, A2=kr22=k(3r1)2=k(9r12)=9(kr12)=9A1A_2 = k r_2^2 = k (3r_1)^2 = k (9r_1^2) = 9 (k r_1^2) = 9 A_1. A2=9×616=5544A_2 = 9 \times 616 = 5544 sq.cm. Therefore, the area of the circle whose radius is 4242 cm is 55445544 sq.cm.