9 Trigonometry; cosine and sine; addition formulae
COMMENT. In this chapter we go on to deal fully with reflex-angles as well as with
wedge and straight ones, we define the cosine and sine of an angle and we deal with
addition of angles. As a vitally convenient aid to identifying the two angles with a
given support|BAC, we start by introducing the notion of the indicator of an angle.
9.1 Indicatorofanangle..........................
9.1.1 .....................................
Definition.Ifαis an angle with support|BAC,we call the other angle with support
|BACtheco-supportedangle forα, and denote it by co−spα.
α
co−spα
Figure 9.1. Co-supported angle.
i(α)
C
A
B
Figure 9.2. Angle indicators.
i(α)
C
A
B
Geometry with Trigonometry
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