The papers 1 and Huanggang 100 are very good, especially the science papers of mathematics and physics, which are typical questions and have more duality than Huanggang 100.
2. Huanggang 100 Volume has strong learning ability, solid foundation and flexible application, which is better than the whole volume of primary school mathematics.
Look at the answer.
The answer is that the cosine of the vector PA and the normal vector N =¡Ì2/4, and the normal vector