The landlord's answer is matching method, and the process is as shown in the figure. In the future, we can use substitution method to make t=x- 1, x = t 1, and f(t) = (T 1) 2, then expand the calculation of f(t), and then replace t with x, which is the demand. This method is relatively simple and mechanical, and easy to use. I hope the landlord will adopt it.