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See solution.
We are asked to show that the product of the slopes of perpendicular lines is - 1.
Let's use the notation on the diagram.
We know that the slope of a line is the quotient of the rise and the run between any two points on the line. Let's use the measurements given on the diagram to write an expression for the slopes.
Points on the Line | Rise | Run | Slope |
---|---|---|---|
A and C | b | a | m_(AC)=b/a |
C and B | - b | c | m_(CB)=- b/c |
We can use these expressions to find the product of the slopes. m_(AC)m_(CB)=b/a*- b/c=-b^2/ac Let's keep this expression in mind. If the lines are perpendicular, then the following statements are also true.
According to Corollary 1 to Theorem 7-3, the length of the altitude is the geometric mean of the lengths of the segments of the hypotenuse. a/b=b/c The Cross Products Property tells us that the product of the means is equal to the product of the extremes. b^2=ac Let's substitute this in the expression for the product of the slopes we got above. m_(AC)m_(CB)=-b^2/ac=-ac/ac=-1 This proves that the product of the slopes of perpendicular lines is - 1.