Definition:Secant/Definition from Circle/Second Quadrant

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Definition

Consider a unit circle $C$ whose center is at the origin of a cartesian coordinate plane.


SecantSecondQuadrant.png


Let $P$ be the point on $C$ in the second quadrant such that $\theta$ is the angle made by $OP$ with the $x$-axis.

Let a tangent line be drawn to touch $C$ at $A = \left({1, 0}\right)$.

Let $OP$ be produced to meet this tangent line at $B$.


Then the secant of $\theta$ is defined as the length of $OB$.


As $OP$ needs to be produced in the opposite direction to $P$, the secant is therefore a negative number in the second quadrant.