A math "thought" question (not homework help)...

Pre-College Issues High School Life
21

<p>Lol @ your teacher…</p>

<p>The only POSSIBLE way I can see you ever losing points for how you solved it is if you “rushed” it a bit.</p>

<p>y = cos (ln x)
dy/dx = -sin(ln x) * d/dx(ln x) –> Missing this step?
dy/dx = -sin(ln x) * (1/x)
dy/dx = -sin(ln x) / x</p>

<p>But even then, I don’t think this was why you got the points off, which in that case, you are correct and your teacher is wrong. My AP Calc BC teacher even showed us how to derive that very equation using chain rule, and there was never any indication as to what your physics teacher was talking about, or for that matter even needing the d/dx part in the 1st step shown above. So yeah, puzzling indeed as to what his thought process was on this…I mean…wouldn’t an AP Calc BC teacher know more about derivatives/chain rule than a physics teacher?</p>

22

<p>Your teacher is wrong. You are right. You simply re-substituted, which is allowed at any time. dy/du is always going to be a function of u, but you simply made u=ln x, so dy/du =-sin(lnx) is still a function of u. I do not understand how your teacher does not understand such a trivial matter lol. You did nothing wrong since you took the derivative then substituted.</p>

23

<p>^^ Well even that’s considered too shorthand for him, lol. He wanted us to do the whole dy/dx = dy/du*du/dx thing.</p>

<p>^ I know, that’s what I thought!</p>

24

<p>Bump…Any other thoughts?</p>

25

<p>Go ask the Calc BC teacher and have him talk to your physics teacher. And watch him get schooled.</p>

26

<p>

That’s a legit request, but doing so is not mutually exclusive with substituting at the time you did.</p>