Posted by **sam** on Friday, May 9, 2008 at 12:43am.

Use implicit differentiation to find the equation of the tangent line to the curve xy3+xy=14 at the point (7,1) . The equation of this tangent line can be written in the form y=mx+b

i dont seem to no how to find m or b

- math -
**JP**, Friday, May 9, 2008 at 1:05am
Take the derivative of both sides of the equation implicitly:

d/dx(xy^3+xy)=d/dx(14)

y^3+3xy^2*y'+y+xy'=0

At x=7, y=1. Thus, 1^3+3(7)(1^2)*y'+1+7y'=0

which yields y'=-2/28=-1/14

The slope of the tangent line is m which also is y'. Thus, you now have m=-1/14. Since the point (7,1) is on the tangent line, then just plug these coordinates as well as the value of m into the equation y=mx+b and solve for b.

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