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Sine
Trigonometry Base
Understand that 'sine' is fundamental in trigonometry, a branch of mathematics dealing with angles and sides of triangles. 
Ratio Concept
'Sine' in mathematics expresses a ratio, not just any number, which helps to understand various geometrical relationships. 
Not Just Triangles
Besides right triangles, 'sine' extends to functions graphed as waves, used in physics, engineering, and signal processing. 