All ACT Math Resources
Example Questions
Example Question #21 : Circles
A circle with a radius of five is centered at the origin. A point on the circumference of the circle has an x-coordinate of two and a positive y-coordinate. What is the value of the y-coordinate?
Recall that the general form of the equation of a circle centered at the origin is:
x2 + y2 = r2
We know that the radius of our circle is five. Therefore, we know that the equation for our circle is:
x2 + y2 = 52
x2 + y2 = 25
Now, the question asks for the positive y-coordinate when x = 2. To solve this, simply plug in for x:
22 + y2 = 25
4 + y2 = 25
y2 = 21
y = ±√(21)
Since our answer will be positive, it must be √(21).
Example Question #21 : How To Find The Equation Of A Circle
Which of the following gives the equation of a circle tangent to the line with its center at ?
The equation for a circle is (x - h)2 + (y - k)2 = r2, where (h,k) is the center of the circle and r is the radius.
We can eliminate (x+3) +(y+2) = 4 and (x+3)2 + (y+2)2 = 64. The first equation does not square the terms in parentheses, and the second refers to a center of (-3,-2) rather than (3,2).
Drawing the line y = -2 and a point at (3,2) for the center of the circle, we see that the only way the line could be tangent to the circle is if it touches the bottom-most part of the circle, directly under the center. The point of intersection will be (3,-2). From this, we can see that the distance from the center to this point of intersection is 4 units between (3,2) and (3,-2). This means the radius of the circle is 4.
Use the center point and the radius in the formula for a circle to find the final answer: (x - 3)2 + (y - 2)2 = 16
Example Question #1 : Graphing
Solve and graph the following inequality:
To solve the inequality, the first step is to add to both sides:
The second step is to divide both sides by :
To graph the inequality, you draw a straight number line. Fill in the numbers from to infinity. Infinity can be designated by a ray. Be sure to fill in the number , since the equation indicated greater than OR equal to.
The graph should look like:
Example Question #201 : Coordinate Geometry
Points and lie on a circle. Which of the following could be the equation of that circle?
If we plug the points and into each equation, we find that these points work only in the equation . This circle has a radius of and is centered at .
Example Question #1 : Graphing
Which of the following lines is perpendicular to the line ?
The key here is to look for the line whose slope is the negative reciprocal of the original slope.
In this case, is the negative reciprocal of .
Therefore, the equation of the line which is perpendicular to the original equation is:
Example Question #201 : Graphing
Let D be the region on the (x,y) coordinate plane that contains the solutions to the following inequalities:
, where is a positive constant
Which of the following expressions, in terms of , is equivalent to the area of D?
Example Question #2 : Graphing
A triangle is made up of the following points:
What are the points of the inverse triangle?
The inverse of a function has all the same points as the original function, except the x values and y values are reversed. The same rule applies to polygons such as triangles.
Example Question #2 : How To Graph Inverse Variation
Electrical power can be generated by wind, and the magnitude of power will depend on the wind speed. A wind speed of (in ) will generate a power of . What is the minimum wind speed needed in order to power a device that requires ?
The simplest way to solve this problem is to plug all of the answer choices into the provided equation, and see which one results in a power of .
Alternatively, one could set up the equation,
and factor, use the quadratic equation, or graph this on a calculator to find the root.
If we were to factor we would look for factors of c that when added together give us the value in b when we are in the form,
.
In our case . So we need factors of that when added together give us .
Thus the following factoring would solve this problem.
Then set each binomial equal to zero and solve for v.
Since we can't have a negative power our answer is .
Example Question #3 : How To Graph Inverse Variation
Compared to the graph , the graph has been shifted:
units to the left.
units down.
units up.
units to the right.
units down.
units to the left.
The inside the argument has the effect of shifting the graph units to the left. This can be easily seen by graphing both the original and modified functions on a graphing calculator.
Example Question #2 : Graphing
The graph of passes through in the standard coordinate plane. What is the value of ?
To answer this question, we need to correctly identify where to plug in our given values and solve for .
Points on a graph are written in coordinate pairs. These pairs show the value first and the value second. So, for this data:
means that is the value and is the value.
We must now plug in our and values into the original equation and solve. Therefore:
We can now begin to solve for by adding up the right side and dividing the entire equation by .
Therefore, the value of is .