2. If the vertical line from A is perpendicular to BC and passes through D, then DC AD can be represented by 300*sin30 300*cos30.

Then you can calculate the length of AB from BD AD (all right triangles are easy to calculate).

3. express b as a and substitute a+b to become (a's square -6a)/(a-8).

This formula is equal to =10+A-8+(16/(a-8)) > =10+8 =18 In this case, a-8 takes 4 a, and 12 b takes 6.

4. Since it is a geometric series and a5= 1, then A4 = 1/A6, A3 = 1/A7, A2 = 1/A8, A 1 = 1/A9, from which we can get

5. Because sinB+cosB= root 2, the sum of their squares is 1. It can be calculated that the angle B is 45 degrees, and sinA= 1/2 can be calculated by sine theorem, so that A=30 degrees.

6. What does this mean again? Operator mean? S5。 S6+ 15=0

7. The topic asks you to find the maximum value of xy, which has nothing to do with x2+y2/2. You can see that the point you found is xy= 12, and the answer is xy= 12.5. There is nothing to say about the process of this problem. Calculate the expressions of three straight lines, and then calculate y directly as a function of X. There is no technical content.