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posted by on .

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

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

  • Algebra - ,

    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,

  • Algebra - ,

    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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