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# Further Differentiation - C4 watch

1. Theres this question i just cant do:

I have attempted it over and over, but cannot get the 2e part of the solution.

xlny + ylnx = 1 find the tangnet at points (1,e)

Use implicit differentiation:

lny + dy/dx (x/y) + y/x + dy/dx(lnx) = 0

dy/dx = (lny + y/x)/ (-x/y - lnx)

Thats as far as i got, then when i subbed the values in, i got the wrong asnwer.

2. did you get the gradient to be -e(e + 1) ?
3. (Original post by mik1w)
did you get the gradient to be -e(e + 1) ?
i got (-e+e²)/-1 = e-e²

I probably messed up. What did you get as the formula for the tangent
4. Thanks... realised i did my working wrong. Your gradient was right.

Thanks!!

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Updated: November 23, 2005
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