Problem Solving

GmatGreen wrote:

In the coordinate system above, which of the following is the equation of line l?

A) 2x - 3y = 6
B) 2x + 3y = 6
C) 3x + 2y = 6
D) 2x - 3y = -6
E) 3x - 2y = -6

We start by defining the equation of line l using the slope-intercept form of a line (y = mx + b), where m = slope and b = the y-intercept.

Notice that the two points (0,2) and (3,0) are on line l. We can use these two points to determine the slope. The formula for slope is:

m = (change in y)/(change in x) or

m = (y_2 - y_1)/(x_2 - x_1)

Plugging in our points we have:

m = (0 - 2)/(3 - 0)

m = -2/3

We also see from the diagram that the y-intercept of line l is 2. Substituting the slope and the y-intercept into the line equation we have:

y = (-2/3)x + 2

The final step is to recognize that the answer choices are in a different form than is our equation for line l. Thus, we have to manipulate our equation such that it will match one of the answer choices. Let's first multiply the entire equation by 3. Doing so gives us:

3y = -2x + 6

Then add 2x to both sides of the equation:

2x + 3y = 6

Alternate solution:

To obtain an equation of a line, we can also use the two-intercept form of a line (x/a + y/b = 1) where a is the x-intercept and b is the y-intercept of the line. This is a lesser-known form of an equation of a line, but it comes in handy when we know or are given the x- and y-intercepts of the line. Here, we see that the x-intercept of the line is 3 and the y-intercept is 2. Thus the equation in the two-intercept form is:

x/3 + y/2 = 1

Multiply the entire equation by 6, and we have:

2x + 3y = 6.

The answer is B

GmatGreen wrote:
In the coordinate system above, which of the following is the equation of line l?
A) 2x - 3y = 6
B) 2x + 3y = 6
C) 3x + 2y = 6
D) 2x - 3y = -6
E) 3x - 2y = -6

We can see that points (0,2) and (3,0) are ON THE LINE. So, their coordinates must SATISFY the equation of the line.

Let's start with (0,2).
(A) 2x - 3y = 6. 2(0) - 3(2) = -6 ELIMINATE
(B) 2x + 3y = 6. 2(0) + 3(2) = 6 KEEP
(C) 3x + 2y = 6. 3(0) +2(2) = 4 ELIMINATE
(D) 2x - 3y = -6. 2(0) - 3(2) = -6 KEEP
(E) 3x - 2y = -6. 3(0) - 2(2) = -4 ELIMINATE

Great. We're down to B or D

Let's test (3,0).
(B) 2x + 3y = 6. 2(3) + 3(0) = 6 KEEP
(D) 2x - 3y = -6. 2(3) - 3(0) = 6 ELIMINATE

Answer: B

Cheers,
Brent