ACT Math : Algebra

Study concepts, example questions & explanations for ACT Math

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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?

Possible Answers:

Correct answer:

Explanation:

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= 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 ?

Possible Answers:

Correct answer:

Explanation:

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:

Possible Answers:

Correct answer:

Explanation:

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:

Number_line

Example Question #201 : Coordinate Geometry

Points  and  lie on a circle. Which of the following could be the equation of that circle?

Possible Answers:

Correct answer:

Explanation:

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 ?

Possible Answers:

Correct answer:

Explanation:

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?

Possible Answers:

Correct answer:

Explanation:

  Inequality_region1

Example Question #2 : Graphing

A triangle is made up of the following points: 

What are the points of the inverse triangle?

Possible Answers:

Correct answer:

Explanation:

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  ?

Possible Answers:

Correct answer:

Explanation:

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:

Possible Answers:

 units to the left.

 units down.

 units up.

 units to the right.

 units down.

Correct answer:

 units to the left.

Explanation:

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 ?

Possible Answers:

Correct answer:

Explanation:

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 .

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