All PSAT Math Resources
Example Questions
Example Question #62 : Circles
A circle with diameter of length is inscribed in a square. Which of the following is equivalent to the area inside of the square, but outside of the circle?
In order to find the area that is inside the square but outside the circle, we will need to subtract the area of the circle from the area of the square. The area of a circle is equal to . However, since we are given the length of the diameter, we will need to solve for the radius in terms of the diameter. Because the diameter of a circle is twice the length of its radius, we can write the following equation and solve for :
Divide both sides by 2.
We will now substitute this into the formula for the area of the circle.
area of circle =
We next will need to find the area of the square. Because the circle is inscribed in the square, the diameter of the circle is equal to the length of the circle's side. In other words, the square has side lengths equal to d. The area of any square is equal to the square of its side length. Therefore, the area of the square is .
area of square =
Lastly, we will subtract the area of the circle from the area of the square.
difference in areas =
We will rewrite so that its denominator is 4.
difference in areas =
The answer is .
Example Question #21 : Radius
An original circle has an area of . If the radius is increased by a factor of 3, what is the ratio of the new area to the old area?
The formula for the area of a circle is . If we increase by a factor of 3, we will increase the area by a factor of 9.
Example Question #14 : Radius
A square has an area of . If the side of the the square is the same as the diameter of a circle, what is the area of the circle?
The area of a square is given by so we know that the side of the square is 6 in. If a circle has a diameter of 6 in, then the radius is 3 in. So the area of the circle is or .
Example Question #71 : Circles
Mary has a decorative plate with a diameter of ten inches. She places the plate on a rectangular placemat with a length of 18 inches and a width of 12 inches. How much of the placemat is visible?
First we will calculate the total area of the placemat:
Next we will calculate the area of the circular place
And
So
We will subtract the area of the plate from the total area
Example Question #22 : Radius
The picture above contains both a circle with diameter 4, and a rectangle with length 8 and width 5. Find the area of the shaded region. Round your answer to the nearest integer
First, recall that the diameter of a circle is twice the value of the radius. Therefore a circle with diameter 4 has a radius of 2. Next recall that the area of a circle with radius is:
The area of the rectangle is the length times the width:
The area of the shaded region is the difference between the 2 areas:
The nearest integer is 27.
Example Question #21 : Radius
Allen was running around the park when he lost his keys. He was running around aimlessly for the past 30 minutes. When he checked 10 minutes ago, he still had his keys. Allen guesses that he has been running at about 3m/s.
If Allen can check 1 square kilometer per hour, what is the longest it will take him to find his keys?
Allen has been running for 10 minutes since he lost his keys at 3m/s. This gives us a maximum distance of from his current location. If we move 1800m in all directions, this gives us a circle with radius of 1800m. The area of this circle is
Our answer, however, is asked for in kilometers. 1800m=1.8km, so our actual area will be square kilometers. Since he can search 1 per hour, it will take him at most 10.2 hours to find his keys.
Example Question #321 : Plane Geometry
A 12x16 rectangle is inscribed in a circle. What is the area of the circle?
10π
90π
50π
120π
100π
100π
Explanation: Visualizing the rectangle inside the circle (corners touching the circumference of the circle and the center of the rectangle is the center of the circle) you will see that the rectangle can be divided into 8 congruent right triangles, with the hypotenuse as the radius of the circle. Calculating the radius you divide each side of the rectangle by two for the sides of each right triangle (giving 6 and 8). The hypotenuse (by pythagorean theorem or just knowing right triangle sets) the hypotenuse is give as 10. Area of a circle is given by πr2. 102 is 100, so 100π is the area.
Example Question #322 : Plane Geometry
A circle is inscribed in a square whose side is 6 in. What is the difference in area between the square and the circle, rounded to the nearest square inch?
The circle is inscribed in a square when it is drawn within the square so as to touch in as many places as possible. This means that the side of the square is the same as the diameter of the circle.
Let
So the approximate difference is in area
Example Question #321 : Plane Geometry
Two equal circles are cut out of a rectangular sheet of paper with the dimensions 10 by 20. The circles were made to have the greatest possible diameter. What is the approximate area of the paper after the two circles have been cut out?
16
43
23
56
43
The length of 20 represents the diameters of both circles. Each circle has a diameter of 10 and since radius is half of the diameter, each circle has a radius of 5. The area of a circle is A = πr2 . The area of one circle is 25π. The area of both circles is 50π. The area of the rectangle is (10)(20) = 200. 200 - 50π gives you the area of the paper after the two circles have been cut out. π is about 3.14, so 200 – 50(3.14) = 43.
Example Question #101 : Geometry
Kate has a ring-shaped lawn which has an inner radius of 10 feet and an outer radius 25 feet. What is the area of her lawn?
325π ft2
525π ft2
175π ft2
275π ft2
125π ft2
525π ft2
The area of an annulus is
where is the radius of the larger circle, and is the radius of the smaller circle.