1. Absolute classic trigonometric function problem: find sin 10sin20…sin90, pay attention to various degrees, it is not easy to print here. Tip: Use the triple angle formula sin3x=4sinxsin(60-x)sin(60+x), and then take x as 10 degree, 20 degree and 30 degree respectively, and multiply the two sides for calculation. 2. Super heuristic plane vector problem: Let A and B be plane vectors, define the outer product of vectors as a*b=|a||b|sin@, and @ is the included angle between A and B (1) If a = (x 1, y 1) and b = (x2, y2). Tip: the proof of the formula of inner product coordinates in imitation of books. (2) Using the above conclusions, it is proved that the necessary and sufficient conditions for the straight lines of vectors A and B * * * are x1y2-x2y1= 0; (3) Knowing the coordinates of the three vertices of the triangle, find the triangle area. Tip: Let A, B and C be the vertices of a triangle, and find the coordinates of vectors AB and AC. Note that the area of triangles A, B and C is 1/2 of the absolute value of the outer product of AB and AC, and then it is obtained by the first vector outer product coordinate formula. PS: If you are interested, you can apply all the deduction methods of inner product to outer product and see what kind of conclusion you will get.