SSAT Upper Level Math : How to find whether lines are perpendicular

Study concepts, example questions & explanations for SSAT Upper Level Math

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Example Questions

Example Question #11 : How To Find Whether Lines Are Perpendicular

Lines 1 and 2, which are perpendicular, have their -intercepts at the point . The -intercept of Line 1 is at the point . Give the -intercept of Line 2.

Possible Answers:

Correct answer:

Explanation:

The slope of a line with -intercept  and -intercept  is . For Line 1, , so Line 1 has slope . The slope of Line 2, which is perpendicular to Line 1, will be the opposite of the reciprocal of this, which is . Setting  equal to this and , we get

, or 

Cross-multiplying:

The -intercept of Line 2 is .

Example Question #12 : How To Find Whether Lines Are Perpendicular

Which of the following choices gives the equations of a pair of perpendicular lines with the same -intercept?

Possible Answers:

 and 

 and 

 and 

 and 

 and 

Correct answer:

 and 

Explanation:

All of the equations are given in slope-intercept form , so we can answer this question by examining the coefficients of , which are the slopes, and the constants, which are the -intercepts. In each case, since the lines are perpendicular, each -coefficient must be the other's opposite reciprocal, and since the lines have the same -intercept, the constants must be equal.

Of the five pairs, only 

 and 

and 

 and 

have equations whose -coefficients are the other's opposite reciprocal. Of these, only the latter pair of equations have equal constant terms. 

 and 

is the correct choice.

Example Question #13 : How To Find Whether Lines Are Perpendicular

Given: the following three lines on the coordinate plane:

Line 1: The line of the equation 

Line 2: The line of the equation 

Line 3: The line of the equation 

Which of the following is a true statement?

Possible Answers:

None of the other responses is correct.

No two of Line 1, Line 2, or Line 3 form a pair of perpendicular lines.

Line 1 and Line 3 are perpendicular; Line 2 is perpendicular to neither.

Line 2 and Line 3 are perpendicular; Line 1 is perpendicular to neither.

Line 1 and Line 2 are perpendicular; Line 3 is perpendicular to neither.

Correct answer:

Line 1 and Line 3 are perpendicular; Line 2 is perpendicular to neither.

Explanation:

The slope of each line can be calculated by putting the equation in slope-intercept form  and noting the coefficient of :

 

Line 1:

Slope of Line 1: 

 

Line 2: 

Slope of Line 2: 

 

Line 3: The equation is already in slope-intercept form; its slope is 2.

 

Two lines are perpendicular if and only their slopes have product . The slopes of Lines 1 and 3 have product ; they are perpendicular. The slopes of Lines 1 and 2 have product ; they are not perpendicular. The slopes of Lines 2 and 3 have product ; they are not perpendicular. 

Correct response: Line 1 and Line 3 are perpendicular; Line 2 is perpendicular to neither.

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