Triangle ABC has been rotated 90° to create triangle DEF. Using the image below, prove that lines BC and EF have the opposite and reciprocal slopes. You must show all of your work to receive credit.

Slope of CB × Slope of FE = 1 × (-1) = -1 [opposite and reciprocal slopes].
Slope of a line = vertical distance along the line / horizontal distance across the line = change in y / change in x.
If a line has a slope of -3, the opposite and reciprocal of -3 is 1/3. Thus, if two slopes are opposite and reciprocals, when multiplied together, we would get -1.
For example, -3 × 1/3 = -1.
Find the slope of CB:
Slope of CB = change in y/change in x = (2 - 5)/(1 - 4)
Slope of CB = (-3)/(-3)
Slope of CB = 1
Find the Slope of FE:
Slope of FE = change in y/change in x = (-1 -(-4))/(2 - 5)
Slope of FE = (3)/(-3)
Slope of FE = -1
Slope of CB × Slope of FE = 1 × (-1) = -1 [opposite and reciprocal slopes].
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