You will need

- - the radius of the semicircle;
- - the range;
- a pair of compasses;
- - a sheet of paper;
- pencil;
- the formula for the area of a circle.

Instruction

1

Construct a circle with a given radius. The center of its label as a: to get a semi-circle, enough to hold through this point cut to the intersection with the circle. This cut is the diameter of this circle and equal to two radii. Remember what circumference and what is the circle. A circle is a line all points of which are removed from the center at the same distance. Circle the part of the plane bounded by this line.

2

Remember the formula for area of a circle. It is equal to the square of the radius multiplied by the constant factor π = 3.14. That is, the area of a circle expressed by the formula S=πR2, where S is the area and R is the radius of the circle. Calculate the area of a semicircle. It is equal to half the area of a circle, i.e. S1= πR2/2.

3

In case you are in conditions given the circumference, find the radius first. The circumference is calculated by the formula P=2nr. Accordingly, to find the radius should be the circumference divided by twice the coefficient. It turns out the formula R=P/2π.

4

The semicircle can be represented as a sector. A sector is a portion of a circle, which is limited by two radii and an arc. The size of the sector is equal to the area of the circle multiplied by the ratio of Central angle to a full angle of a circle. That is, in this case it is expressed by the formula S=π*R2*n°/360°. The sector angle is known, it is 180°. Substituting its value, you'll get the same formula is S1= πR2/2.

Note

There are tasks, where the arc angle is not in degrees but in radians. In this case, you must use the conversion formula Ar = Ad *π / 180°, where Ar is the angle in radians, and Ad — he is in degrees. To compute the area of a semicircle is not particularly important. Even if you are a semi-circle as a sector, in the final formula no degree no. But it may be necessary to compute the area of the sector that has a different Central angle.

In some problems it is required to find the area of a circle or semicircle, constructed on a particular side right or wrong polygon. Without additional constructions in this case can not do. It is necessary to divide a given shape into the other, whose parameters you specified, or you can easily find them. After that, calculate the desired direction, which often represents the diameter of a circle or semicircle.

In some problems it is required to find the area of a circle or semicircle, constructed on a particular side right or wrong polygon. Without additional constructions in this case can not do. It is necessary to divide a given shape into the other, whose parameters you specified, or you can easily find them. After that, calculate the desired direction, which often represents the diameter of a circle or semicircle.

# Advice 2 : How to find the area, if you know the diameter

Knowing only the length

**of the diameter**of the circle, you can calculate not only**square**the circle but also the square of some other geometrical figures. This follows from the fact that the diameters of the inscribed or circumscribed around the shapes of the circles coincide with the lengths of their sides or diagonals.Instruction

1

If you need to find

**the area**of a circle (S) for the length of its**diameter**(D), multiply the number PI (π) on the squared length**of the diameter**, and the result divide by four: S=2 π*D2/4. For example, if the diameter of the circle is twenty inches, then its**area**can be calculated as: 3,142 * 202 / 4 = 9,86 * 400 / 4 = 986 square centimeters.2

If you need to find

**the area**of a square (S) on the diameter described around the circumference (D), construct the length**of diameter**square, and the result is split in half: S=D2/2. For example, if the diameter of the circumscribed circle is twenty inches, then**the area**of square can be calculated as: 202 / 2 = 400 / 2 = 200 square centimeters.3

If

**the area**of a square (S) you need to find the diameter of the inscribed circle (D), it is enough to build the length**of the diameter**in square: S=D2. For example, if the diameter of the inscribed circle is twenty inches, then**the area**of square can be calculated as: 202 = 400 square centimeters.4

If you need to find

**the area**of a right triangle (S) according to the known**diameter**of the inscribed m (d) and described (D) circles around it, then build the length**of the diameter of**the inscribed circle in the square and divide by four, and to the result add half the product of the lengths of the diameters of the inscribed and circumscribed circles: S=d2/4 + D*D/2. For example, if the diameter of the circumscribed circle is twenty inches, and inscribed to ten centimeters, then**the area**of the triangle can be calculated as: 102 / 4 + 20*10/2 = 25 + 100 = 125 square centimeters.5

Use the built in Google calculator to perform the necessary calculations. For example, to calculate a search

**area**of a right triangle according to the example of the fourth step, it is necessary to enter a search query: "10^2 / 4 + 20*10/2", and then press the Enter key.# Advice 3 : How to measure the area of a circle

A circle is a simple geometrical figure that has no corners. If you measure the distance from the center

**of the circle**to any extreme point, it will always be equal to the radius. The task typically requires to calculate the diameter and find**the area****of a circle**. These indicators are easy to calculate if the radius**of the circle**is known.You will need

- calculator.

Instruction

1

To find the area

**of the circle**first, its radius lift to a square that is in the second degree. And then multiply the result by the number π (PI). If the problem is the radius given the diameter of the figure, can be split into 2. Now obtained by dividing the radius of the shapes use for the convenience of calculating the area**of a circle**.2

To find the value of the square of the radius

**of a circle**use a calculator. To start enter the value of radius**of circle**, then find a special button with the symbol x2. This symbol on the button shows that number to be raised to the second power. If you have any difficulty, multiply the radius**of the circle**on itself. To find the**area****of a circle**, can use and diameter. The radius is ½ the diameter, and hence it can be represented as a fraction where the numerator is the value of the diameter, and in the denominator - 2. When computing the square of this fraction on a calculator, take the value of the diameter**of the circle**in the second degree, and then divide the resulting number by 4.3

The value of the square of the radius

**of the circle**, multiply by a factor of π (PI). To find the**area****of a circle**, you can use a more exact or rounded value. To do this, enter the corresponding number (3,1415926535897932384626433832795 or 3.14). Often it is possible to use a special button marked with the symbol π (PI), as in many models of calculators.4

The area

**of a circle is**measure in square units. If the radius was given in centimeters (cm),**area**is expressed in square centimeters (cm2). When calculating radius from a diameter**of the circle**unit of measure does not change. For example, if the diameter were given in inches, then the radius will be measured in inches, and the desired**area**will be obtained in inches square. The problem is not always immediately specified radius. Sometimes initially given diameter**of the circle**. If you ignore this and use the diameter for calculations instead of the radius, the inevitable errors in calculations. To find the radius, the size of the diameter divide by 2.Useful advice

To calculate the area of a circle use the number π (PI) is approximately equal 3,1415926535897932384626433832795. If greater accuracy is not required, you can round this factor to 3.14.

# Advice 4 : How to find the area, knowing the diameter

Objectives to calculate the area of a circle often meet in the school geometry course. To find the

**area**of a circle, you need to know the length**of the diameter**or radius of a circle in which it is enclosed.You will need

- - the length of the diameter of the circle.

Instruction

1

A circle is a figure in the plane consisting of a plurality of points disposed in the same distance from another point called the centre. A circle is a flat geometric figure, represents the set of points enclosed in a circle which is the boundary of the circle. Diameter is a segment that connects two points on the circle and passing through its center. The radius is the segment connecting a point on the circle and its center. π — the number "PI", a mathematical constant that is the constant. It shows the ratio of the circumference to the length of its

**diameter**. To compute the exact value of π is impossible. In geometry use the approximate value of this number: π ≈ 3,142

The area of a circle equals the product of the square of the radius to the number calculated by the formula: S=NR^2, where S is

**the area**of the circle, R is the length of the radius of the circle.3

From the definition of the radius, it follows that it is equal to half

**the diameter**. Hence, the formula becomes: S=π(D/2)^2, where D is the length**of the diameter**of the circle. Substitute in the formula the value**of the diameter**, calculate**the area**of a circle.4

The area of a circle is measured in square units — mm2, cm2, m2, etc. In what units is expressed you received

**the area**of a circle depends on what units was the diameter of a circle.5

If you need to calculate

**the area**of a ring use this formula: S=π(R-R)^2, where R, r are radii of the outer and inner circumferences of the ring, respectively.Useful advice

There is an international day of "PI", which is celebrated on March 14. The exact time of onset of the official date — 1 hour 59 minutes 26 seconds, according to figures the number is 3,1415926...