### 4Some trigonometry and geometry of triangles and circles

This chapter reminds you of what trig is for, and how it works in triangles. It also explains some of the special geometrical properties of triangles and circles, because they may be very useful to you in applications of maths to your own special subject area.

The chapter is divided into the following sections.

**4.A Trigonometry in right-angled triangles**

(a) Why use trig ratios?

(b) Pythagoras’ Theorem,

(c) General properties of triangles

(d) Triangles with particular shapes,

(e) Congruent triangles – what are they, and when?

(f) Matching ratios given by parallel lines,

(g) Special cases – the sin, cos and tan of 30°, 45° and 60°,

(h) Special relations of sin, cos and tan

**4.B Widening the ...**