在Rt△ABC中,∠C=90°,∠A、∠B、∠C的对边分别为a、b、c,则sinA=a/c叫做∠A的正弦,cosA=b/c叫做∠A的余弦,tanA=a/b叫做∠A的正切。∠A的正弦、余弦、正切都叫做锐角A的三角函数。一个锐角的三角函数是用直角三角形的边的比值来定义的,当一个锐角的大小确定后,它的三角函数值就确定了,不管这个锐角是单独的一个角,还是在某个三角形中。因此,在求一个锐角的三角函数值时,若是特殊角,我们就用特殊角 In Rt △ ABC, ∠C = 90 °, 对A, ∠B, 对C on the opposite side were a, b, c, then sinA = a / c is called the sine of ∠A, cosA = b / c is called ∠ A cosine, tanA = a / b is called the tangent of ∠A. The sine, cosine and tangent of ∠A are all called trigonometric functions of acute angle A. An acute trigonometric function is defined by the ratio of the sides of a right-angled triangle. When the size of an acute corner is determined, its trigonometric value is determined, whether the acute angle is a single corner or a triangle. Therefore, in seeking a trigonometric triangular function value, if the special angle, we use a special angle