According to the general method of setting a straight line, the linear equation can be written as y-0=k(x- 1) and can be solved by substitution. But the question says that AB does not coincide with the left and right vertices, which means that K is not equal to 0. If AB is parallel to the y axis, the slope does not exist, and this formula cannot be expressed, so set it.

X=my+ 1, on the one hand, it is convenient to replace X with Y, on the other hand, it also considers the situation that the slope does not exist.

If you are satisfied, please accept it.