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# Parametric equations, differentiating watch

1. Hey, there's a question in the MEI text book which asks me to differentiate

x = 1/(1+t) which was fine, I got

dx/dt = 1/(1+t)^2

But i can't do,

y=1/(1+t)(1-t)

dy/dt=2t/(1+t)^2(1-t)^2

sorry it looks so confusing! but if anyone could help me differentiate dy/dt, that would be great help! thanks
2. Don't have a pen and paper to hand to work it out, but I would use the chain rule with u=(1+t)(1-t) and then the product rule to differentiate u.
3. Your first differentiation is wrong, it should be

For the second one, use partial fractions:

4. Yeah, firstly you have 1/1-t^2 when expanded.
using quotient rule you then have 0- (-2t)/(1-t^2)^2
and 1-t^2 = (1-t)(1+t) so
dy/dt = 2t/(1-t)^2(1+t)^2 as required

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Updated: March 18, 2011
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