Rapid Recall
- For the function $f(x,y) = 4x^2e^{5y}$, compute both $f_y$ and $\dfrac{\partial ^2f }{\partial x\partial y}$.
Solution
$f_y = 4x^2(5e^{5y}) = 20x^2e^{5y}$ and $\dfrac{\partial ^2 f}{\partial x\partial y} = (4\cdot 2x)(5e^{5y})=40xe^{5y}$.
- Let $f(x,y)=x^2+y$, and $g(x,y)=2x+y$. Solve the system $\vec \nabla f = \lambda \vec \nabla g$ and $g(x,y)=3$.
Solution
We have $\vec \nabla f = (2x,1)$, $\vec \nabla g = (2,1)$. The equation $\vec \nabla f = \lambda \vec \nabla g$ gives us $2x=\lambda\cdot 2$ and $1 = \lambda 1$. The second equation tells us $\lambda =1$, and the first equation tells us $x=\lambda=1$. Substitution into $2x+y=3$ tells us $y=1$.
- The surface $x^2+3y^2-4z=-5$ passes through the point $P=(2,1,3)$. Give an equation of the tangent plane to this surface at $P$. Hint: Use differentials.
Solution
Differentials tell us $2xdx+3ydy-4dz=0$. We know $x=2$, $y=1$, and $z=3$. We also know that if $Q=(x,y,z)$ is another point on the plane, then the change from $P$ to $Q$ is $dx = x-2$, $dy=y-1$, and $dz=z-3$. Substitution (plug it in, plug it in) gives the equation of the tangent plane as $$2(3)(x-3)+3(1)(y-1)-4(z-3)=0.$$
Group problems
- The differential of $f(x,y)=x^2+4y$ is $df = 2xdx+4dy$. At the point $(3,-2)$ this differential is $df = 6dx+4dy$. This differential is sometimes called a "linearization" of the function.
- Consider the level curve of $f$ that passes through $(3,-2)$. Give an equation of the tangent line to this curve at $(3,-2)$. [What are $df,dx,dy$ if we move to a point $(x,y)$ on the level curve that is really close to $(3,-2)$?]
- Consider the surface plot of $f$. Give an equation of the tangent plane to this surface at the point $(3,-2,f(3,-2))$. [What are $df,dx,dy$ if we move to a point $(x,y,z)$ on the surface that is really close to $(3,-2,f(3,-2))$?]
- A rover travels along the line $g(x,y)=2x+3y=6$. The surrounding terrain has elevation $f(x,y)=x^2+4y$. The rover reaches a local minimum along this path, and our job is to find the location of this minimum.
- Compute $\vec \nabla f$ and $\vec \nabla g$.
- Write the system of equations that results from $\vec \nabla f=\lambda\vec \nabla g$ together with $g(x,y) = 6$.
- Solve the system above (you should get $x=4/3$ and $y=10/9$).
- Consider the function $f(x,y,z) = 3xy+z^2$. We'll be analyzing the surface at the point $P=(1,-3,2)$.
- Compute the differential $df$, and then the differential at $P$.
- For a level surface, the output remains constant (so $df=0$). If we let $(x,y,z)$ be a point on the surface really close to $P$, then we have $dx=x-1$, $dy=y-(-3)$ and $dz = z-?$. Plug this information into the differential to obtain the differential at $P$ to obtain an equation of the tangent plane.
- Give an equation of the tangent plane to the level surface of $f$ that passes through $(1,2,-3)$.
- Give an equation of the tangent plane to the level surface of $f$ that passes through $(a,b,c)$.
- What relationship exists between the gradient of $f$ at $P$ and the tangent plane through $P$?
- Suppose a plane passes through the point $(a,b,c)$ and has normal vector $(A,B,C)$. Give an equation of that plane.
- Give an equation of the tangent plane to $xy+z^2=7$ at the point $P=(-3,-2,1)$.
- Give an equation of the tangent plane to $z=f(x,y)=xy^2$ at the point $P=(4,-1,f(4,-1))$.
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