Trigonometry 三角学
Radians and Degrees 弧度和角度subtend V-TA line or chord subtends an angle or arc. 一条弧/弦 对向/形成 一个角/弧度角A
Radians and Degrees 弧度和角度
- subtend V-T
A line or chord subtends an angle or arc. 一条弧/弦 对向/形成 一个角/弧度角 - A radian (弧度) is a unit used to measure an angle, equal to the angle at the center of a circle whose arc (弧,弦) is the same length as the circle’s radius (半径). 1 radian is equivalent to 57.296 degrees and
radians equals a right angle (直角).
The length of the arc that subtends this angle tells us how many radiuses the angle covers.
对应于角的弧长告诉我们该角占据了多少个半径长度。
- circumference /səˈkʌmfərəns/ N-UNCOUNT 周长
The circumference of a circle, place, or round object is the distance around its edge. - halfway ADV 在中间地
Halfway means in the middle of a place or between two points, at an equal distance from each of them.
e.g. If we go halfway around a circle, we'll call it one pi radians.
π radians = 180 degrees, so 
Hence, 
The more examples for the problem, the better it will become a little bit intuitive to convert degrees to radians.
Trigonometry 三角学
We call a circle a unit circle (单位圆). It means that the circle has a radius (半径) of 1.
A positive angle (正角) would move in a counterclockwise direction (逆时针方向). A negative angle (反角) would move in a clockwise direction (顺时针方向).
The hypotenuse (斜边,弦) of a right-angled triangle (直角三角形) is the side opposite its right angle. It is also called the hypotenuse ray, and abbreviated as hyp. These two legs of the angle are adjacent (邻边) and opposite (对边). They are also abbreviated as adj and opp.
The terminal side (终边) of the angle intersects the unit circle at the point
The cosine of θ is the change of x, that is 
Pythagorean Trig Identity 毕达哥拉斯三角恒等式
- A quadrant (扇形,四分之一圆周,象限) is one of four equal parts into which a circle or other shape has been divided.
e.g. ...Quadrant I. 第一象限
Sinusoidal Function 正弦函数
- midline 中线, 中间线
amplitude 振幅, 幅度
period 周期
难以记住的三角函数公式
- Double-Angle Identities
- Half-Angle Identities
