Jiāoliúdiàn de diànyā suí shíjiān àn zhèngxián guīlǜ biànhuà.
The voltage of alternating current rises and falls over time like a sine wave.
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The other basic functions are 余弦 (cosine) and 正切 (tangent). 弦 means "chord": the sine began as half a chord measured in a circle. The 正弦定理 is the law of sines.
