SAT Math : Coordinate Geometry

Study concepts, example questions & explanations for SAT Math

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Example Questions

Example Question #45 : New Sat Math No Calculator

What is the value of the -intercept for the line given below?

 

Possible Answers:

Correct answer:

Explanation:

The x-intercept is where the line crosses the x-axis. In other words, 

This gives:

Subtracting 98 from both sides gives:

Dividing both sides by 14 gives the final answer:

Example Question #602 : Geometry

Give the area of the triangle on the coordinate plane that is bounded by the -axis, and the lines of the equations  and 

Possible Answers:

Correct answer:

Explanation:

It is necessary to find the vertices of the triangle, each of which is a point at which two of the three lines intersect.

The intersection of the -axis - the line  - and the line of the equation , is found by noting that if , then, by substitution, ; this point of intersection is at , the origin. 

The intersection of the -axis and the line of the equation  is found similarly:

This intersection point is at .

The intersection of the lines with equations  and  can be found using the substitution method, setting  in the latter equation and solving for :

Since , making  the point of intersection.

The vertices are at .

The lines in question are graphed below, and the triangle they bound is shaded:

Triangle z

If we take the horizontal side as the base, its length is seen to be the -coordinate of the -intercept, ; its (vertical) height is the -coordinate of the opposite vertex, . The area is half the product of the two, or

Example Question #161 : Geometry

Given the line , what is the sum of the -intercept and the -intercept?

Possible Answers:

Correct answer:

Explanation:

Intercepts occur when a line crosses the -axis or the -axis. When the line crosses the -axis, then  and .  When the line crosses the -axis, then  and . The intercept points are  and . So the -intercept is  and the  intercept is  and the sum is .

Example Question #22 : How To Find X Or Y Intercept

Where does the line given by y=3(x-4)-9 intercept the -axis?

Possible Answers:

Correct answer:

Explanation:

First, put in slope-intercept form. 

y=3x-21

To find the -intercept, set  and solve for .

Example Question #21 : How To Find X Or Y Intercept

Where does the graph of 2x + 3y = 15 cross the x-axis?

Possible Answers:

(-7.5, 0)

(0, 5)

(0, 0)

(7.5, 0)

(0, -5)

Correct answer:

(7.5, 0)

Explanation:

To find the x-intercept, set y=0 and solve for x. This gives an answer of x = 7.5.

Example Question #1 : How To Find The Equation Of A Circle

If the center of a circle is at (0,4) and the diameter of the circle is 6, what is the equation of that circle?

Possible Answers:

x2 + y2 = 9

(x-4)2 + y2 = 9

x2 + (y-4)2 = 9

x2 + (y-4)2 = 36

(x-4)2 + y2 = 36

Correct answer:

x2 + (y-4)2 = 9

Explanation:

The formula for the equation of a circle is:

(x-h) 2 + (y-k)2 = r2

Where (h,k) is the center of the circle.

h = 0 and k = 4

and diameter = 6 therefore radius = 3

(x-0) 2 + (y-4)2 = 32

x2 + (y-4)2 = 9

Example Question #1 : How To Find The Equation Of A Circle

Circle A is given by the equation (x – 4)2 + (y + 3)2 = 29. Circle A is shifted up five units and left by six units. Then, its radius is doubled. What is the new equation for circle A?

Possible Answers:

(x – 2)2 + (y + 2)2 = 58

(x – 10)2 + (y + 8)2 = 116

(x + 2)2 + (y – 2)2 = 116

(x – 10)2 + (y + 8)2 = 58

(x + 2)2 + (y – 2)2 = 58

Correct answer:

(x + 2)2 + (y – 2)2 = 116

Explanation:

The general equation of a circle is (x – h)2 + (y – k)2 = r2, where (h, k) represents the location of the circle's center, and r represents the length of its radius. 

Circle A first has the equation of (x – 4)2 + (y + 3)2 = 29. This means that its center must be located at (4, –3), and its radius is √29.

We are then told that circle A is shifted up five units and then left by six units. This means that the y-coordinate of the center would increase by five, and the x-coordinate of the center would decrease by 6. Thus, the new center would be located at (4 – 6, –3 + 5), or (–2, 2).

We are then told that the radius of circle A is doubled, which means its new radius is 2√29.

Now, that we have circle A's new center and radius, we can write its general equation using (x – h)2 + (y – k)2 = r2.

(x – (–2))2 + (y – 2)2 = (2√29)2 = 22(√29)2 = 4(29) = 116.

(x + 2)2 + (y – 2)2 = 116.

The answer is (x + 2)2 + (y – 2)2 = 116.

Example Question #1 : How To Find The Equation Of A Circle

Which of the following equations describes all the points (x, y) in a coordinate plane that are five units away from the point (–3, 6)?

Possible Answers:

(x – 3)2 + (y + 6)2 = 25

(x + 3)2 + (y – 6)2 = 25

(x – 3)2 – (y + 6)2 = 25

y + 6 = 5 – (x – 3)2

(x – 3)2 + (y + 6)2 = 5

Correct answer:

(x + 3)2 + (y – 6)2 = 25

Explanation:

We are trying to find an equation for all of the points that are the same distance (5 units) from (–3, 6). The locus of all points equidistant from a single point is a circle. In other words, we need to find an equation of a circle. The center of the circle will be (–3, 6), and the radius, which is the distance from (–3,6), will be 5. 

The standard form of a circle is given below:

(x – h)2 + (y – k)2 = r2, where the center is located at (h, k) and r is the length of the radius.

In this case, h will be –3, k will be 6, and r will be 5.

(x – (–3))2 + (y – 6)2 = 52

(x + 3)2 + (y – 6)2 = 25

The answer is (x + 3)2 + (y – 6)2 = 25.

Example Question #1 : How To Find The Equation Of A Circle

What is the equation for a circle of radius 12, centered at the intersection of the two lines:

y1 = 4x + 3

and

y2 = 5x + 44?

Possible Answers:

(x - 41)2 + (y - 161)2 = 144

(x - 22)2 + (y - 3)2 = 12

(x - 3)2 + (y - 44)2 = 144

None of the other answers

(x + 41)2 + (y + 161)2 = 144

Correct answer:

(x + 41)2 + (y + 161)2 = 144

Explanation:

To begin, let us determine the point of intersection of these two lines by setting the equations equal to each other:

4x + 3 = 5x + 44; 3 = x + 44; –41 = x

To find the y-coordinate, substitute into one of the equations.  Let's use y1:

y = 4 * –41 + 3 = –164 + 3 = –161

The center of our circle is therefore: (–41, –161).

Now, recall that the general form for a circle with center at (x0, y0) is:

(x - x0)2 + (y - y0)2 = r2

For our data, this means that our equation is:

(x + 41)2 + (y + 161)2 = 122 or (x + 41)2 + (y + 161)2 = 144

Example Question #1 : Circles

The diameter of a circle has endpoints at points (2, 10) and (–8, –14). Which of the following points does NOT lie on the circle?

Possible Answers:

(2, –14)

(–15, –7)

(–8, 10)

(9,3)

(–8,–12)

Correct answer:

(–8,–12)

Explanation:

Circle_point1

 

Circle_point2

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