Trigonometry worksheets

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Trigonometric ratios

Grade 9

Write sine, cosine and tangent as fractions of a right triangle's sides

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A simple way through trigonometry

It begins with the definition, before any calculator. A trig ratio is a fraction of a right triangle's sides — sin A is the opposite over the hypotenuse, read straight off the figure — and because every triangle here is a Pythagorean triple, the ratio stays an exact fraction. That is the ground the rest stands on: once the ratio is a known thing, solving a right triangle uses it to find a missing side or angle.

Why this order works

The unit circle is the year's memory work. Degrees and radians relate by one fact — a straight angle is 180° and also π radians — so every conversion is a single multiplication, with the answer left in terms of π. The exact values at the special angles come off two triangles, kept as surds rather than rounded, because a decimal hides the √3/2 a student is meant to recognise. The identities that follow, sin²+cos²=1 and the cofunction rule, tie the ratios of an angle together.