The area of a circle varies directly as the square of its radius. A circle with radius 2 in. has area 12.568 in. squared. What is the area of a circle with radius 6.2 ​in.?

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Part 1
The area of a circle with radius 6.2 in. is approximately
  
120.634
in. squared .
​(Simplify your answer. Round to the nearest thousandth as​ needed.)

1 answer

The area \( A \) of a circle varies directly as the square of its radius \( r \). This can be expressed with the formula:

\[ A = k r^2 \]

where \( k \) is the constant of proportionality. Given that the area of a circle with radius 2 inches is 12.568 square inches, we can first find \( k \):

\[ 12.568 = k (2^2) \]

\[ 12.568 = k (4) \]

\[ k = \frac{12.568}{4} = 3.142 \]

Now that we have the value of \( k \), we can use it to find the area of a circle with a radius of 6.2 inches:

\[ A = k r^2 \] \[ A = 3.142 (6.2^2) \]

Calculating \( 6.2^2 \):

\[ 6.2^2 = 38.44 \]

Now, substituting back into the area formula:

\[ A = 3.142 \times 38.44 \]

Calculating this gives:

\[ A \approx 120.63408 \]

Rounding this to the nearest thousandth, the area of a circle with a radius of 6.2 inches is approximately:

\[ A \approx 120.634 \text{ in}^2 \]