Question

Determine the missing information in the paragraph proof.

Given: Line PQ is rotated 90° counterclockwise to form line P’Q’. The lines are perpendicular. Line PQ contains the points (a, b) and (c, d). Line P’Q’ contains the points (–b, a) and (–d, c).

Prove: The slopes of perpendicular lines are negative reciprocals.

On a coordinate plane, 2 perpendicular lines are shown. Line P Q has points (a, b) and (c, d). Line P prime Q prime has points (negative b, a) and (negative d, c).

The slopes of lines PQ and P’Q’ can be determined using the formula m = StartFraction v 2 minus v 1 Over x 2 minus x 1 EndFraction

The product of these slopes is ________. This product shows that the slopes are negative reciprocals. It is given that the lines are perpendicular and we have shown that the slopes of the lines are negative reciprocals.

(StartFraction d minus b Over c minus a EndFraction) (StartFraction c minus a Over negative d + b EndFraction) = negative 1
(StartFraction b minus a Over a + b EndFraction) (StartFraction c minus d Over negative d minus c EndFraction) = negative 1
(StartFraction d minus b Over c minus a EndFraction) (StartFraction c minus a Over negative d + b EndFraction) = 1
(StartFraction b minus a Over a + b EndFraction) (StartFraction c minus d Over negative d minus c EndFraction) = 1

Answer 1

The product is -1.

Compute:
m_PQ = (d − b)/(c − a), m_P'Q' = (c − a)/(−d + b) = (c − a)/(b − d).
So m_PQ · m_P'Q' = (d − b)/(c − a) · (c − a)/(b − d) = (d − b)/(b − d) = −1.

Thus the correct choice is the first option:
((d − b)/(c − a))((c − a)/(−d + b)) = −1, showing the slopes are negative reciprocals.