If anyone within 1 mile of the singer can hear her, that means there is a circle centered at the singer of radius 1 mile, and anyone within that circle can hear the singer. The area of that circle is π r^2 = π (1)^2 = π

After the microphone is adjusted, there is a circle of radius 2 miles around the singer, and anyone within that circle can hear her. The area of this new circle is π r^2 = π (2)^2 = 4π

The increase in area is just found by subtracting: 4π - π = 3π
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