Posted by Leanna on Wednesday, January 26, 2011 at 3:10pm.
Consider the curve given by x^2+4y^2=7+3xy
a) Show that dy/dx=(3y2x)/(8y3x)
b) Show that there is a point P with xcoordinate 3 at which the line tangent to the curve at P is horizontal. Find the ycoordinate of P.
c) Find the value of d^2y/dx^2 (second derivative) at the point P found in part b). Does the curve have a local maximum, a local minimum, or neither at the point P? Justify your answer..
The only part that I did was a). I don't really know how to find Point P for b) and c)

AP Calculus  Damon, Wednesday, January 26, 2011 at 3:37pm
a)
2 x dx + 8 y dy = 3x dy + 3y dx
dy(3x8y)=dx(2x3y)
b)
Where is the slope = 0?
where 2x = 3y
if x = 3
9 + 4 y^2 = 7 + 9 y
4 y^2  9 y + 2 = 0
(4y1)(y2) = 0
y = 1/4 or y = 2
check both of those to find which works
(3,2)
3*2  2*3 = 0
yes, (3,2) works
(3,1/4)
3/4 6 = 0 no way so (3,2) is it
c)
dy/dx=(3y2x)/(8y3x)
d^2y/dx^2 = [(8y3x)(2)(3y2x)(3)] / (8y3x)^2
= [(169)(2)]/(169)^2 = 2/7
that is negative, so a maximum of the function

AP Calculus  Leanna, Wednesday, January 26, 2011 at 3:47pm
Thank you so much! It makes perfect sense and when I calculated it, it matched up.

AP Calculus  Brian, Thursday, January 12, 2012 at 10:24pm
I'm confused as to how to do part C), I don't remember how to find the second derivative of an implicit differentiation problem!
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