# SSAT Upper Level Math : How to find the area of a circle

## Example Questions

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### Example Question #1 : Geometry

Give the area of the figure in the above diagram.

Explanation:

The figure is a sector of a circle with radius 8; the sector has degree measure . The area of the sector is

### Example Question #1 : Area And Circumference Of A Circle

Give the area of the above figure.

Explanation:

The figure is a semicircle - one-half of a circle - with radius 5.5, or . Its area is one-half of the square of the radius multiplied by  - that is,

### Example Question #2 : Area And Circumference Of A Circle

A circle on the coordinate plane has equation

.

Which of the following gives the area of the circle?

Explanation:

The equation of a circle on the coordinate plane is

,

where  is the radius. Therefore, in this equation,

.

The area of a circle is found using the formula

,

so we substitute 66 for , yielding

.

### Example Question #3 : Area And Circumference Of A Circle

Give the area of a circle that circumscribes a 30-60-90 triangle whose shorter leg has length 11.

Explanation:

If a right triangle is inscribed inside a circle, then the arc intercepted by the right angle is a semicircle, making the hypotenuse of triangle a diameter.

The length of a hypotenuse of a 30-60-90 triangle is twice that of its short leg, so the hypotenuse of this triangle will be twice 11, or 22. The diameter of the circle is therefore 22, and the radius is half this, or 11. The area of the circle is therefore

### Example Question #5 : Geometry

Give the ratio of the area of a circle that circumscribes an equilateral triangle to that of a circle that is inscribed inside the same triangle.

Explanation:

Examine the following diagram:

If a (perpendicular) radius of the inscribed circle is constructed to the triangle, and a radius of the circumscribed circle is constructed to a neighboring vertex, a right triangle is formed. By symmetry, it can be shown that this is a 30-60-90 triangle, and, subsequently,

If we let , the area of the inscribed circle is .

Then , and the area of the circumscribed circle is

The ratio of the areas is therefore 4 to 1.

### Example Question #6 : Geometry

Give the area of a circle that circumscribes an equilateral triangle with perimeter 54.

The correct answer is not among the other responses.

Explanation:

An equilateral triangle of perimeter 54 has sidelength one-third of this, or 18.

Construct this triangle and its circumscribed circle, as well as a perpendicular bisector to one side and a radius to one of that side's endpoints:

Each side of the triangle has measure 18, so . Also, the triangle formed by the segments, by symmetry, is a 30-60-90 triangle. By the 30-60-90 Theorem,

and .

The latter is the radius, so the area of this circle is

### Example Question #4 : Area And Circumference Of A Circle

central angle of a circle has a chord with length 7. Give the area of the circle.

The correct answer is not among the other responses.

Explanation:

The figure below shows , which matches this description, along with its chord :

By way of the Isosceles Triangle Theorem,  can be proved equilateral, so . This is the radius, so the area is

### Example Question #8 : Geometry

Give the area of a circle that circumscribes a  triangle whose longer leg has length .

Explanation:

If a right triangle is inscribed inside a circle, then the arc intercepted by the right angle is a semicircle, making the hypotenuse of the triangle a diameter.

By the 30-60-90 Theorem, the length of the shorter leg of a 30-60-90 triangle is that of the longer leg divided by , so the shorter leg will have length ; the hypotenuse will have length twice this length, or

.

The diameter of the circle is therefore ; the radius is half this, or . The area of the circle is therefore

### Example Question #5 : Area And Circumference Of A Circle

central angle of a circle has a chord with length . Give the area of the circle.

Explanation:

The figure below shows , which matches this description, along with its chord :

By way of the Isoscelese Triangle Theorem,  can be proved a 45-45-90 triangle with hypotenuse 15. By the 45-45-90 Theorem, its legs, each a radius, have length that can be determined by dividing this by :

The area is therefore

### Example Question #6 : Area And Circumference Of A Circle

Give the area of a circle that is inscribed in an equilateral triangle with perimeter .

Explanation:

An equilateral triangle of perimeter 72 has sidelength one-third of this, or 24.

Construct this triangle and its inscribed circle, as well as a radius to one side - which, by symmetry, is a perpendicular bisector - and a segment to one of that side's endpoints:

Each side of the triangle has measure 24, so . Also, the triangle formed by the segments, by symmetry, is a 30-60-90 triangle. Therefore,

which is the radius of the circle. The area of this circle is

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