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Mathematics>Standard Multiple Choice

Read the following SAT test question and then click on a button to select your answer.

Which of the following equations represents a line in the x y-plane with x-intercept at (-15,0) and y-intercept at (0,-9)?

A.x/15+y/9=1
B.-x/15-y/9=1
C.x/9-y/15=1
D.-x/9+y/15=1

答案和解析

答案：B

解析：

Choice B is correct. Using the points given, first find the slope of the line:

-9-0/0-(-15) =-3/5. The y-intercept of the line is –9, so the equation of the line can be written as y=(-3/5)x-9.

Since none of the answer choices are in this form, the equation must be rewritten. First, add (3/5)x to both sides of the equation, resulting in (3/5)x+y=-9. Next, divide both sides of the equation by –9, which yields (3/5)/-9x+y/(-9)=9/-9 which can be simplified to -x/15-y/9=1.