Common Core suggests: "Understand that by similarity, side ratios in right triangles are properties of the angles in the triangle, leading to definitions of trigonometric ratios for acute angles.".
Find Sin, Cos, Tan from Lengths of Triangle Sides - Expii
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4-03 Right Triangle Trigonometry
Find Sin, Cos, Tan from Lengths of Triangle Sides - Expii
What Is the Unit Circle? - Expii
Find Sin, Cos, Tan from Lengths of Triangle Sides - Expii
Right Triangle Trigonometry
Right Triangle Trigonometry
Right Triangle Trigonometry Review