dana.king

Help,

My teacher assigned an extra assignment for our class, to help boost
our grades for University application. Unfortunately, I have been
unable to solve one of the problems. My teacher gave us the fully
simplified answer, but I have been unable to get the same answer.

The question reads: Given: 4x^2 - 5y^2 = -1, find d^2y/dx^2 Simplify
Fully.

The answer is: 4/25 y^3 <----How do i get this?

Any help with this solution would be greatly, appreciated.

Thank you

julian v. noble

It is not surprising that you can't get the answer above. It is
wrong. The correct answer is

y'' = (4/25) y^{-3}

I suggest you differentiate the original relation w.r.t. x, solve
for y', differentiate again, and eliminate y' and x using the
original expression and the expression for y' . The rest is algebra.

--
Julian V. Noble
Professor Emeritus of Physics
jvn@lessspamformother.virginia.edu
^^^^^^^^^^^^^^^^^^
http://galileo.phys.virginia.edu/~jvn/

"For there was never yet philosopher that could endure the
toothache patiently."

-- Wm. Shakespeare, Much Ado about Nothing. Act v. Sc. 1.

dana.king

href="http://galileo.phys.virginia.edu/~jvn/">http://galileo.phys.virginia.edu/~jvn/</a>


HELP!
What does w.r.t. mean? And could you show me the work for this
question

david w. cantrell

It means "with respect to".


I'm sure he could, as could I. But this is a problem that _you_ are
supposed to do. JVN has already provided you with a nice outline of
the necessary steps; you should be able to do the rest yourself if
you are to deserve the extra credit.

David

julian v. noble

^^^^^^

"with respect to" -- it's a standard abbreviation.


When Hell freezes over. Try to act a little bit
grateful when someone helps you, even if you don't
really feel that way.

Nobody here really gives a Tinker's damn (look it
up!) whether you get extra credit or not. Some of
us might care whether you learn something, however,
since we would hate to fly a plane, or cross a bridge
that you might design, if you don't learn a little math. ;-)


--
Julian V. Noble
Professor Emeritus of Physics
jvn@lessspamformother.virginia.edu
^^^^^^^^^^^^^^^^^^
http://galileo.phys.virginia.edu/~jvn/

"You catch more flies with honey than with vinegar."

-- one of my Grandma's sayings. Take it to heart.

ken pledger

Right. I saw the question under a different title, and probably
gave too much help before finding this thread. It may be too late to
matter anyway.

Ken Pledger.