Linear algebra - Proof of a thesis concerning the height in triangles!

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My Math teacher gave us some tasks we should work on. I solved most of them already, however I still could not manage to figure out the solution for this one! I would really appreciate, if someone of you could give me a hint or the name of this mathematic problem, so i can research in the Internet. Here is the task!

Proof that the following sentence is true for right-angeled triangles:

h²= |AC|²-|AC|²*(cos(a))²

This is looks almost like the triangle from the task

I should mention, that this task is taken from the field of linear algebra, so |AC| and h are vectors!

Thank you for your support!

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we get $$AC^2=(1-\cos(a)^2)=AC^2\sin(a)^2$$ and this is true since $$\sin(a)=\frac{h}{AC}$$