In chapter 11 angles were described in terms of degrees and in chapter 12 circular measure was introduced. There is a third method which describes an acute angle by the ratios of the sides of a right-angled triangle containing the angle. They are the trigonometrical ratios and at first we shall consider only three of them, the sine the cosine and the tangent of an angle.
KeywordsAcute Angle Trigonometrical Identity Isosceles Trapezium Natural Sine Circular Measure
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