<Ul> <Li> Two lines are parallel if and only if they are not the same line (coincident) and either their slopes are equal or they both are vertical and therefore both have undefined slopes . Two lines are perpendicular if the product of their slopes is − 1 or one has a slope of 0 (a horizontal line) and the other has an undefined slope (a vertical line). </Li> <Li> The angle θ between − 90 ° and 90 ° that a line makes with the x-axis is related to the slope m as follows: </Li> </Ul> <Li> Two lines are parallel if and only if they are not the same line (coincident) and either their slopes are equal or they both are vertical and therefore both have undefined slopes . Two lines are perpendicular if the product of their slopes is − 1 or one has a slope of 0 (a horizontal line) and the other has an undefined slope (a vertical line). </Li> <Li> The angle θ between − 90 ° and 90 ° that a line makes with the x-axis is related to the slope m as follows: </Li> <Dl> <Dd> <Dl> <Dd> m = tan ⁡ (θ) (\ displaystyle m = \ tan (\ theta)) </Dd> </Dl> </Dd> <Dd> and <Dl> <Dd> θ = arctan ⁡ (m) (\ displaystyle \ theta = \ arctan (m)) (this is the inverse function of tangent; see inverse trigonometric functions). </Dd> </Dl> </Dd> </Dl>

The slope of a function curve or graph definition