There is a clever solution to this problem. Look at this question: in fact, you can do this. First fix M0 and N0 (that is, the points corresponding to M and N when P is the diagonal midpoint), and then connect M0B, M0D 1, N0B, N0D 1. In this plane quadrilateral (actually a diamond), determine the change of the size of the triangle BMN with the current diagonal X.

You don't have to list any equations. In the first half (in the range before the midpoint), x increases uniformly, while the base and height of the triangle increase linearly at the same time. The area is the product of the two and increases in a square relationship (such as the graphic change of y=x*x). In the second half, x increases uniformly and the length of triangle decreases linearly. In fact, your main difference is that the area decreases quickly and slowly. Maybe you think it's a multiplication of two numbers. When we know that the closer two numbers are, the greater the product. Obviously, the closer these two numbers are to the smaller increment (otherwise there is no maximum). The reason is that the gap between the two (the length and width of the triangle) is getting bigger and bigger now. The closer we get, the slower we will be, and then the faster we will be, so we can be sure that the answer is B.

As for why you said Mn = 2pm = 2pb * tan∠MBP = (2 ∠ 6)/3 * x, this is very simple. In this diamond, Tan ∠ MBP is a constant value, namely = (2 ∠ 6)/3, so ah Mn = 2pm = 2pb * Tan ∠ MBP. .

I hope you can accept it.