I wish there was some documentation on Algebraic Formulas. I guess I still don't understand how it works.

Here is a trimmed down version of a problem where I get stuck and I don't know why. Suppose this is what I want:

And these are the defined variables:

I want algebraic formulas entered as _0 and _1, and I want to give partial credit, so I entered:

I have no idea what is wrong. I figured the program would run through the variables t, compute p1+t*v1 and compare it with _0 etc. Since I don't know what the syntax is, and I cannot find any documentation on this, except for a few examples, I can only try to mimic the examples, and then blindly try different thing.

Here are some of the variations I tried:

Believe me, I have tried many other things. The last two months I have been trying to learn moodleformulas, and it is like one big black box with hundreds of buttons, and I am just trying different buttons to see what will happen.

I would have included the XML of all this (except it doesn't let me save it).

I can only include the XML of the version where I changed it to Numerical Formula.

Any help would be appreciated.