Question 20: If B is used as BD, the vertical AC and AC intersect at point D, the angle ABC is 75 degrees, and the angle BCA is 45 degrees. In the isosceles right triangle BCD, the root 2 of BD=CD= 15 can be found by pythagorean theorem, and in the right triangle BAD, the square root of angle A=60 degrees and AD = 5 can be found by trigonometric function, so AC=CD+AD.

Question 25: Divide the right side of the equation into (x-1)/(x+1) = (mx+m+n)/(x+1). As can be seen from the above equation, the denominator on both sides is the same, so the numerator must be the same, that is; X- 1=MX+M+N, because this equation is about X, we can get X=MX and-1=M+N by comparison.