PSAT Math : How to find out if a point is on a line with an equation

Study concepts, example questions & explanations for PSAT Math

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Example Questions

Example Question #82 : Coordinate Geometry

In the xy -plane, line is given by the equation 2x - 3y = 5. If line passes through the point (a ,1), what is the value of a ?

Possible Answers:
5
4
-2
3
-1
Correct answer: 4
Explanation:

The equation of line l relates x -values and y -values that lie along the line. The question is asking for the x -value of a point on the line whose y -value is 1, so we are looking for the x -value on the line when the y-value is 1. In the equation of the line, plug 1 in for and solve for x:

2x - 3(1) = 5

2x - 3 = 5

2x = 8

x = 4. So the missing x-value on line l is 4.

Example Question #25 : Coordinate Geometry

The equation of a line is: 2x + 9y = 71

Which of these points is on that line?

Possible Answers:

(4,-7)

(-4,7)

(2,7)

(-2,7)

(4,7)

Correct answer:

(4,7)

Explanation:

Test the difference combinations out starting with the most repeated number.  In this case, y = 7 appears most often in the answers.  Plug in y=7 and solve for x.  If the answer does not appear on the list, solve for the next most common coordinate.

2(x) + 9(7) = 71

2x + 63 = 71

2x = 8

x = 4

Therefore the answer is (4, 7)

Example Question #26 : Coordinate Geometry

Which of the following lines contains the point (8, 9)?

Possible Answers:

\dpi{100} \small 3x+6=2y\(\displaystyle \dpi{100} \small 3x+6=2y\)

\dpi{100} \small 3x-6=2y\(\displaystyle \dpi{100} \small 3x-6=2y\)

\dpi{100} \small 8x=9y\(\displaystyle \dpi{100} \small 8x=9y\)

\dpi{100} \small 3x+6=y\(\displaystyle \dpi{100} \small 3x+6=y\)

\dpi{100} \small 8x+9=y\(\displaystyle \dpi{100} \small 8x+9=y\)

Correct answer:

\dpi{100} \small 3x-6=2y\(\displaystyle \dpi{100} \small 3x-6=2y\)

Explanation:

In order to find out which of these lines is correct, we simply plug in the values \dpi{100} \small x=8\(\displaystyle \dpi{100} \small x=8\) and \dpi{100} \small y=9\(\displaystyle \dpi{100} \small y=9\) into each equation and see if it balances.

The only one for which this will work is \dpi{100} \small 3x-6=2y\(\displaystyle \dpi{100} \small 3x-6=2y\)

Example Question #1 : Points And Distance Formula

\dpi{100} \small 5x+25y = 125\(\displaystyle \dpi{100} \small 5x+25y = 125\)

Which point lies on this line?

Possible Answers:

\dpi{100} \small (1,4)\(\displaystyle \dpi{100} \small (1,4)\)

\dpi{100} \small (1,5)\(\displaystyle \dpi{100} \small (1,5)\)

\dpi{100} \small (5,1)\(\displaystyle \dpi{100} \small (5,1)\)

\dpi{100} \small (5,5)\(\displaystyle \dpi{100} \small (5,5)\)

\dpi{100} \small (5,4)\(\displaystyle \dpi{100} \small (5,4)\)

Correct answer:

\dpi{100} \small (5,4)\(\displaystyle \dpi{100} \small (5,4)\)

Explanation:

\dpi{100} \small 5x+25y = 125\(\displaystyle \dpi{100} \small 5x+25y = 125\)

Test the coordinates to find the ordered pair that makes the equation of the line true:

\dpi{100} \small (5,4)\(\displaystyle \dpi{100} \small (5,4)\)

\dpi{100} \small 5 (5) + 25 (4) = 25 + 100 = 125\(\displaystyle \dpi{100} \small 5 (5) + 25 (4) = 25 + 100 = 125\)

\dpi{100} \small (1,5)\(\displaystyle \dpi{100} \small (1,5)\)

\dpi{100} \small 5(1)+25(5)= 5+125=130\(\displaystyle \dpi{100} \small 5(1)+25(5)= 5+125=130\)

\dpi{100} \small (5,1)\(\displaystyle \dpi{100} \small (5,1)\)

\dpi{100} \small 5(5)+25(1)= 25+25=50\(\displaystyle \dpi{100} \small 5(5)+25(1)= 25+25=50\)

\dpi{100} \small (5,5)\(\displaystyle \dpi{100} \small (5,5)\)

\dpi{100} \small 5(5)+25(5)= 25+125=150\(\displaystyle \dpi{100} \small 5(5)+25(5)= 25+125=150\)

\dpi{100} \small (1,4)\(\displaystyle \dpi{100} \small (1,4)\)

\dpi{100} \small 5(1)+25(4)= 5+100=105\(\displaystyle \dpi{100} \small 5(1)+25(4)= 5+100=105\)

Example Question #4 : How To Find Out If A Point Is On A Line With An Equation

Points D and E lie on the same line and have the coordinates \(\displaystyle \left ( 1,2\right )\)and \(\displaystyle \left ( 5,7\right )\), respectively.  Which of the following points lies on the same line as points D and E?

Possible Answers:

\(\displaystyle \left ( 2,3.5\right )\)

\(\displaystyle \left ( 2,4\right )\)

\(\displaystyle \left ( 3,4\right )\)

\(\displaystyle \left ( 3,3.25\right )\)

\(\displaystyle \left ( 3,4.5\right )\)

Correct answer:

\(\displaystyle \left ( 3,4.5\right )\)

Explanation:

The first step is to find the equation of the line that the original points, D and E, are on.  You have two points, so you can figure out the slope of the line by plugging the points into the equation 

\(\displaystyle slope = \frac{\Delta y}{\Delta x} = \frac{y_{2}-y_{1}}{x_{2}-x_{1}}\).  

\(\displaystyle slope = \frac{7-2}{5-1} = \frac{5}{4}\)

Therefore, you can get an equation in the line in point-slope form, which is

\(\displaystyle y-2 = 5/4(x-1)\).  

Plug in the answer options, and you will find that only the point \(\displaystyle \left ( 3,4.5\right )\) solves the equation.

Example Question #5 : How To Find Out If A Point Is On A Line With An Equation

Which of the following points is on the line given by the equation \(\displaystyle y=\frac{1}{2}x+3\)?

Possible Answers:

\(\displaystyle (-1, 2)\)

\(\displaystyle (-2,0)\)

\(\displaystyle (1,3)\)

\(\displaystyle (2,4)\)

\(\displaystyle (3,4)\)

Correct answer:

\(\displaystyle (2,4)\)

Explanation:

In order to solve this, try each of the answer choices in the equation:

\(\displaystyle y=\frac{1}{2}x+3\)

For example, when we try (3,4), we find:

\(\displaystyle 4=\frac{1}{2}(3)+3\)

\(\displaystyle 4=1.5+3\)

\(\displaystyle 4=4.5\)

This does not work. When we try all the choices, we find that only (2,4) works:

\(\displaystyle y=\frac{1}{2}x+3\)

\(\displaystyle 4=\frac{1}{2}(2)+3\)

\(\displaystyle 4=1+3\)

\(\displaystyle 4=4\)

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