Geometry with Trigonometry

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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http://dx.doi.org/10.1016/B978-0-12-805066-8.50009-4