Exams and entrance tests
Bring the question you got wrong, and the tutor locates the first step that failed.
MathsPhysicsChemistryProgrammingEntrance tests and olympiads
Every assistant will mark a question. The difference is what runs behind each step of this one.
“I differentiated y = sin(x²) and got cos(x²). Is that right?”
See an example
The first step that failed
NodeStudy · illustrative answer
It is not, and the error sits in one step. You differentiated sin(u) correctly and stopped before differentiating u itself. Here u = x², so the derivative of sin(u) is cos(u)·u′, with u′ = 2x. The answer is 2x·cos(x²).
The principle: the chain rule does not disappear when the inner function is simple. It disappears when you treat the inner function as though it were x itself.
What runs behind it
- The wrong step is not the model’s impression. A deterministic checker refutes the transition, and only a refutation counts: one point of disagreement proves inequality, while agreeing at several proves nothing.
- The notebook inherits the turn’s evidence: verified when a checker refuted something there, provisional when the diagnosis was the model’s. The refutation covers the turn, not each sentence: the page claims the correction locates the step that failed, not that every line was proved by a tool.
- The drill order is the server’s, not the model’s: blocks of different topics alternate where they can, and the faded pair stays together on purpose, because the worked example has to still be fresh when the next problem blanks its key step.
Challenge
A line through (0, 2) is tangent to the curve y = x³ − 3x. Find the point of tangency and justify why there is no other.