Ah, college students come here to ask questions. . . Too professional questions are generally unknown to Baidu. . .

First of all, you should understand that the bottom of binary phase diagram is solid, the top is liquid, and the middle is solid-liquid mixing. This sentence is nonsense, ignore it.

Then the composition of a point on the binary phase diagram can be read directly (except the solid-liquid mixed state). This sentence is also nonsense, continue to ignore it.

Then, the solid-liquid mixing state, such as the composition of point O, depends on how many solid components there are and how many liquid components there are.

So, can the composition of point O be represented by point A and point B? With a and b as axes a and b, the component of point o is not a×ob+b×0a=a×xxS+b×xxL. Right?

Finally, just write in the components A and B, a=A×BxL+B×AxL, b=A×BxS+B×AxS.

There is still a little bit left, Qo×Ax, making trouble by yourself and merging similar items.