Posted by **sharon** on Friday, July 11, 2014 at 8:19pm.

y = -7x - 9, -3y = 21x + 7

without graphing, tell whether the graphs of each pair of equations are perpendicular.

- Algebra -
**Barry**, Friday, July 11, 2014 at 8:33pm
Dear sharon,

the first equation

y = -7x - 9

slope= -7

y-intercept= -9

the second equation

-3y = 21x + 7

y = -7x -(7/3)

slope= -7

y-intercept= -(7/3)

To determine whether the two lines are perpendicular

slope of equation 1 X slope of equation 2 must be -1.

But in this case,

slope of equation 1 X slope of equation 2 = 49 instead of -1

so the two lines are not perpendicular.

(However, slope of equation 1 = slope of equation 2 and the y-intercept of the two equations are not the same, so we know that the two lines are parallel to each other.)

Hope it helps!

Wishing you the best of luck,

Barry

- Algebra -
**Steve**, Friday, July 11, 2014 at 8:34pm
the slopes are the same, so they are either parallel or the same line.

If you multiply the 1st by -3, the constant term is different from 7, so the lines are parallel.

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