How to Find the Area of a Semicircle
A semicircle is half of a circle. Therefore, to find the area of a semicircle, you just have to find the area of a full circle and then divide it by two. It will be faster than you think.
Steps

Find the radius of the semicircle. You'll need the radius to find the area of the semicircle. Let's say the radius of the semicircle is 5 centimeter (2.0 in).
 If you're only given the diameter of the circle, you can divide it by two to get the radius. For example, if the diameter of the circle is 10 centimeter (3.9 in), then you can divide it by 2 (10/2) to get 5 centimeter (2.0 in) as the radius.

Find the area of the full circle and divide it by two. The formula for finding the area of a full circle is πr, where "r" represents the radius of the circle. Since you're finding the area of a semicircle, you'll be looking for half of the area of a circle, which means you have to use the formula for finding the area of a semicircle and then divide it by two. So, the formula you'll have to use to find the area of a semicircle is πr/2. Now, just plug "5 centimeter (2.0 in)" into the formula to get your answer. You can either use the closest approximation for π with your calculator, substitute 3.14 for π, or you can just leave the symbol in place. Here's how you do it:
 Area = (πr)/2
 Area = (π x 5 cm x 5 cm)/2
 Area = (π x 25 cm)/2
 Area = (3.14 x 25 cm)/2
 Area = 39.25 cm

Remember to state your answer in units squared. Since you're finding the area of a shape, you'll have to use units square d (such as cm) in your answer to indicate that you're working with a twodimensional object. If you're calculating volume, then you'll be working with cubic units (such as cm).
Tips
 The area of a semicircle is (1/2)(pi)(r^2).
 The area of a circle is (pi)(r^2)
Warnings
 Be careful not to use the diameter in the formula for finding area! If the diameter is given, then remember to divide it by 2 to get the radius.
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