1, cosine theorem is B2 = A2+C2-2accosb; Note: Angle B is the included angle between side A and side C. ..

2. Multiplication and factorization are A2-B2 = (a+b) (a-b) A3+B3 = (a+b) (A2-AB+B2) A3-B3 = (A-B (A2+AB+B2)).

3. We call the angle formed by counterclockwise rotation a positive angle and the angle formed by clockwise rotation a negative angle.

4. The vertex of the angle coincides with the coordinate origin, and the starting edge of the angle coincides with the positive semi-axis of the shaft. In this way, in which quadrant does the terminal edge of the angle fall, so we say that the angle is in which quadrant (if the terminal edge of the angle falls on the coordinate axis, it does not belong to any quadrant).

5. The central angle of an arc with a length equal to the radius is called 1 radian, and the unit is rad, which is pronounced radian. This system of measuring angles in radians is called radian system.