- #1
Happiness
- 679
- 30
The derivative for the parametric equations ##x=f(t)## and ##y=g(t)## is given by
##\frac{dy}{dx}=\frac{\Big(\frac{dy}{dt}\Big)}{\Big(\frac{dx}{dt}\Big)}##
The proof of the above formula requires that ##y## be a function of ##x##, as seen in http://tutorial.math.lamar.edu/Classes/CalcII/ParaTangent.aspx
But somehow the formula works even if ##y## is not a function of ##x##, for example, it works for the unit circle ##x^2+y^2=1##, whose parametric equations are ##x=\cos t## and ##y=\sin t##.
Does the formula always work as long as the relation between ##x## and ##y## can be parametrised even if ##y## is not a function of ##x##? If not, what is a counterexample? For what parametric equations does the formula fail?
Note: A function is a relation that passes the vertical-line test.
Edit: We exclude cases where the curve intersects itself because the derivative doesn't exist at the intersecting points.
##\frac{dy}{dx}=\frac{\Big(\frac{dy}{dt}\Big)}{\Big(\frac{dx}{dt}\Big)}##
The proof of the above formula requires that ##y## be a function of ##x##, as seen in http://tutorial.math.lamar.edu/Classes/CalcII/ParaTangent.aspx
But somehow the formula works even if ##y## is not a function of ##x##, for example, it works for the unit circle ##x^2+y^2=1##, whose parametric equations are ##x=\cos t## and ##y=\sin t##.
Does the formula always work as long as the relation between ##x## and ##y## can be parametrised even if ##y## is not a function of ##x##? If not, what is a counterexample? For what parametric equations does the formula fail?
Note: A function is a relation that passes the vertical-line test.
Edit: We exclude cases where the curve intersects itself because the derivative doesn't exist at the intersecting points.
Last edited: