====The Calculus of Functions of Several Variables====

===Answers for selected problems===

**Chapter 3**

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**Section 3.1**

3. (a) $ \displaystyle{\lim_{(x,y) \to (2,1)}\left(3xy + x^2y + 4\right) = 14}$

(b) $ \displaystyle{\lim_{(x,y,z) \to (1,2,1)}\frac{3xyz}{2xy^2+4z} = \frac{1}{2}}$

%%(c)%% $ \displaystyle{\lim_{(x,y) \to (2,0)}\frac{\cos(3xy)}{\sqrt{x^2 + 1}} = \frac{1}{\sqrt{5}}}$

(d) $ \displaystyle{\lim_{(x,y,z) \to (2,1,3)}ye^{2x-3y+z} = e^4}$

4. (a) $ \displaystyle{\lim_{(x,y) \to (0,0)}\frac{xy^2}{x^2+y^2} = 0}$

%%(c)%% The limit does not exist:  For example, if we let
\[
f(x,y) = \frac{x}{x+y^2},
\]
$\alpha(t) = (0, t)$, and $ \beta(t) = (t, 0) $, then
\[
\lim_{t \to 0}f(\alpha(t)) = 0,
\]
while
\[
\lim_{t \to 0}f(\beta(t)) = 1.
\]

(e) $ \displaystyle{\lim_{(x,y) \to (0,0)}\frac{1- e^{-(x^2 + y^2)}}{x^2 + y^2} = 1}$

8. (a)  Open

(b) Closed

%%(c)%% Open

(d) Open

(e) Closed

(f) Closed

(g) Open

(h) Neither open nor closed

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**Section 3.2**

1. $ \displaystyle{D_{\mathbf{u}}f(3, 1) = \frac{26}{\sqrt{5}}}$

2. (a) $ \displaystyle{f_x(x,y) = \frac{4y^2-4x^2}{(x^2+y^2)^2}}$;  $ \displaystyle{f_y(x,y) = -\frac{8xy}{(x^2+y^2)^2}}$

%%(c)%% $ \displaystyle{f_x(x,y,z) = 6xy^3z^4-26xy}$

$ \displaystyle{f_y(x,y,z) = 9x^2y^2z^4-13x^2}$

$ \displaystyle{f_z(x,y,z) = 12x^2y^3z^3}$

(e) $ \displaystyle{g_w(w,x,y,z) = \frac{w\cos\left(\sqrt{w^2+x^2+2y^2+3z^2}\right)}{\sqrt{w^2+x^2+2y^2+3z^2}}}$

$ \displaystyle{g_x(w,x,y,z) = \frac{x\cos\left(\sqrt{w^2+x^2+2y^2+3z^2}\right)}{\sqrt{w^2+x^2+2y^2+3z^2}}}$

$ \displaystyle{g_y(w,x,y,z) = \frac{2y\cos\left(\sqrt{w^2+x^2+2y^2+3z^2}\right)}{\sqrt{w^2+x^2+2y^2+3z^2}}}$

$ \displaystyle{g_z(w,x,y,z) = \frac{3z\cos\left(\sqrt{w^2+x^2+2y^2+3z^2}\right)}{\sqrt{w^2+x^2+2y^2+3z^2}}}$

3. (a) $ \displaystyle{\nabla f(x,y,z) = \frac{1}{\sqrt{x^2+y^2+z^2}}(x,y,z)}$

%%(c)%% $ \displaystyle{\nabla f(w,x,y,z) = \frac{1}{1 + (4w+3x+5y+z)^2}(4,3,5,1)}$

4. (a) $ \displaystyle{D_{\mathbf{u}}(-2,1) = -\frac{56}{\sqrt{13}}}$

%%(c)%% $ \displaystyle{D_{\mathbf{u}}(-2,2,1) = -\frac{1}{9\sqrt{6}}}$

5. (a) $ \displaystyle{D_{\mathbf{u}}f(-2,1) = \frac{12}{\sqrt{13}}}$, where $ \displaystyle{\mathbf{u} = \frac{1}{\sqrt{13}}(2,3)}$

%%(c)%%  $ \displaystyle{D_{\mathbf{u}}f(2,1,-1,2) = \frac{4}{\sqrt{15}}}$, where $ \displaystyle{\mathbf{u} = \frac{1}{\sqrt{15}}(1,-1,2,3)}$

6. (a) $ \displaystyle{-20\sqrt{10}e^{-1}}$

(b) Direction: $ \displaystyle{\frac{1}{\sqrt{5}}(-1, 2)}$

Rate of change: $ \displaystyle{40\sqrt{5}e^{-1}}$

%%(c)%% Direction: $ \displaystyle{\frac{1}{\sqrt{5}}(1, -2)}$

Rate of change: $ \displaystyle{-40\sqrt{5}e^{-1}}$

9. (b) $ \displaystyle{\frac{\partial}{\partial y}d_A(x,y) > 0}$, $ \displaystyle{\frac{\partial}{\partial x}d_B(x,y) > 0}$

%%(c)%%  $ \displaystyle{\frac{\partial}{\partial y}d_A(x,y) < 0}$, $ \displaystyle{\frac{\partial}{\partial x}d_B(x,y) < 0}$

11. (a) $ \displaystyle{D_{\mathbf{-u}}f(\mathbf{c}) = -D_{\mathbf{u}}f(\mathbf{c})}$

(b) No

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**Section 3.3**

1. (a) $ A(x,y) = 12x + 4y - 12 $

%%(c)%% $ A(x,y) = 0 $

(e) $ A(w,x,y,z) = 2w + 4x - 12y - 4z -19 $

2. (a) $ 8x - 2y - z = 5 $

%%(c)%% $ 4x - 4y + z = 17 $

4. (a) $ A(x,y,z) = 8x + 12y + 6z - 48 $

(b) $ 26h $

5. $ \displaystyle{\left. \frac{dT}{dt}\right|_{t=0} = 0} $; $ \displaystyle{\left. \frac{dT}{dt}\right|_{t=\frac{\pi}{4}} = 140e^{-\frac{1}{2}}}$; $ \displaystyle{\left. \frac{dT}{dt}\right|_{t=\frac{\pi}{2}} = 0}$

7. $ \displaystyle{\left. \frac{dw}{dt}\right|_{t=\frac{\pi}{3}} = -\frac{96\pi}{16\pi^2+9} \approx -1.807}$

9. (a) $ 2x + y = 5 $

%%(c)%% $ x + 4y = -4 $

10. (a)  $ 2x + y - 3z = 14 $

%%(c)%% $ x + 2y - 2z = 1 $

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**Section 3.4**

1. (a) $ \displaystyle{\frac{\partial^2}{\partial x \partial y}f(x,y) = 6x^2 + 24e^{-3y}}$

%%(c)%% $ \displaystyle{\frac{\partial^2}{\partial x^2}f(x,y) = 6xy^2 - 8e^{-3y}}$

(e) $ \displaystyle{\frac{\partial^3}{\partial x \partial y^2}f(x,y) = 6x^2 - 72xe^{-3y}}$

(g) $ \displaystyle{f_{yy}(x,y) = x^3 - 36x^2e^{-3y}}$

2. (a) $ \displaystyle{\frac{\partial^2}{\partial z \partial x}f(x,y,z) = \frac{2yz(3x^2 - y^2 - z^2)}{(x^2 + y^2 + z^2)^3}}$

%%(c)%% $ \displaystyle{\frac{\partial^2}{\partial z^2}f(x,y,z) = \frac{2xy(3z^2 - x^2 - y^2)}{(x^2 + y^2 + z^2)^3}}$

(e) $ \displaystyle{\frac{\partial^3}{\partial z \partial y \partial x}f(x,y,z) =\frac{2z(3x^4-18x^2y^2+3y^4+2x^2z^2+2y^2z^2-z^4)} {(x^2+y^2+z^2)^4}}$

3. (a)  $ \displaystyle{Hf(x,y) = \begin{bmatrix}6y & 6x - 12y^2 \\ 6x - 12y^2 & -24xy \end{bmatrix}}$

(b) $ \displaystyle{Hf(x,y,z) = \begin{bmatrix}0 & 8yz^3 & 12y^2z^2 \\ 8yz^3 & 8xz^3 & 24xyz^2\\ 12y^2z^2 & 24xyz^2 & 24xy^2z\end{bmatrix}}$

4. (a) $ \displaystyle{P_2(x,y) = x - xy}$

%%(c)%% $ \displaystyle{P_2(x,y) = \frac{1}{2} - \frac{1}{4}(x - 1) - \frac{1}{4}(y - 1)}$

$ \displaystyle{+ \frac{1}{8}(x-1)^2 + \frac{1}{4}(x-1)(y-1) + \frac{1}{8}(y-1)^2}$

5. (a) Positive definite

%%(c)%% Negative definite

(e) Positive definite

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**Section 3.5**

1. Maximum value of $ \dfrac{1}{2} $ at $ \displaystyle{\left(\frac{1}{\sqrt{2}}, \frac{1}{\sqrt{2}}\right)}$ and $ \displaystyle{\left(-\frac{1}{\sqrt{2}}, -\frac{1}{\sqrt{2}}\right)}$; minimum value of $ -\dfrac{1}{2} $ at $ \displaystyle{\left(-\frac{1}{\sqrt{2}}, \frac{1}{\sqrt{2}}\right)}$ and $ \displaystyle{\left(\frac{1}{\sqrt{2}}, -\frac{1}{\sqrt{2}}\right)}$

3. Maximum value of $ 10 $ at $ \left(\sqrt{2}, \sqrt{2}\right)$ and  $ \left(-\sqrt{2}, -\sqrt{2}\right) $; minimum value of $ -2 $ at $ \left(-\sqrt{2}, \sqrt{2}\right)$ and $ \left(\sqrt{2}, -\sqrt{2}\right) $

5. Local minimum of $ \phantom{}0 $ at all points of the form $ (0, y) $, $ -\infty < y < \infty $; local maximum of $ e^{-1} $ at $ (1, 0) $ and $ (-1, 0) $

7. Local maximum of $ 1 $ at $ (1, 1) $ and $ (-1, -1) $; saddle point at $ (0, 0) $

9. Local minimum of $ \phantom{}0 $ at $ (0, 0, 0) $

11. $ 10\hbox{ meters} \times 10\hbox{ meters} \times 10\hbox{ meters} $

12.  $ 8.43\hbox{ meters} \times 8.43\hbox{ meters} \times 8.43\hbox{ meters} $

13. Maximum value of $ \sqrt{3} $ at $ \displaystyle{\left(\frac{1}{\sqrt{3}}, \frac{1}{\sqrt{3}}, \frac{1}{\sqrt{3}}\right)} $; minimum value of $ -\sqrt{3} $ at  $ \displaystyle{\left(-\frac{1}{\sqrt{3}}, -\frac{1}{\sqrt{3}}, -\frac{1}{\sqrt{3}}\right)}$

15. Minimum distance of $ \dfrac{2\sqrt{21}}{7} $ at $ \displaystyle{\frac{2}{7}(2, 4, 1)} $

17. Hottest point: $ 98.28^\circ $ at $ \left(\sqrt{2}, 0, -\sqrt{2}\right) $; coldest point: $ 41.72^\circ $ at $ \left(-\sqrt{2}, 0, \sqrt{2}\right) $

20. Local minimum of $ \phantom{}0 $ at all points of the form $ (x, x) $, $ -\infty < x < \infty $

24. $ y = 9.23x + 114.72 $

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**Section 3.6**

1. (a) $ \displaystyle{\int_1^3 \int_0^2 3xy^2dydx = 32}$

%%(c)%% $ \displaystyle{\int_{-2}^2 \int_{-1}^1 (4 - x^2y^2)dxdy = \frac{256}{9}}$

2. (a) $ \displaystyle{\int\int_D (y^2 - 2xy)dxdy = -\frac{4}{3}}$

%%(c)%% $ \displaystyle{\int\int_D ye^{-x}dxdy = 2(1 - e^{-1}}$

3. (a) $ \displaystyle{\int_0^2 \int_0^y (xy^2 - x^2)dxdy = \frac{16}{15}}$

%%(c)%% $ \displaystyle{\int_0^2 \int_0^{\sqrt{4-x^2}} (4 - x^2 - y^2)dydx = 2\pi}$

5. $ \dfrac{32}{3} $

7. $ \displaystyle{\int\int_D e^{-x^2}dxdy = \frac{1}{2}(1 - e^{-1})}$

9. $ 56\pi $

11. (a) $ \displaystyle{\int_1^2 \int_0^3 \int_{-2}^2 (4 - x^2 - z^2)dydxdz = -16}$

%%(c)%% $ \displaystyle{\int_0^4 \int_0^x \int_0^{x+y} (x^2 - yz)dzdydx = \frac{2432}{15}}$

12. $ 16\pi $

14. (a) Mass: $ \dfrac{16}{3} $; center of mass: $ \displaystyle{\left(1, \frac{1}{2}, 1\right)}$

(b) Mass: $ \dfrac{16}{3} $; center of mass: $ \displaystyle{\left(\frac{4}{5}, \frac{2}{5}, \frac{8}{5}\right)}$

16. (a) $ \dfrac{1}{6} $

(b) $ \dfrac{1}{6} $

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**Section 3.7**

1. $ 2\pi $

3. $ \dfrac{40\pi}{3} $

5. (a) $ \displaystyle{\left(\sqrt{2}, \frac{\pi}{4}\right)}$

%%(c)%% $ \left(\sqrt{10}, 4.3906\right) $

6. (a) $ (3, 0) $

%%(c)%% $ (-5, 0) $

7. $ \displaystyle{\int\int_D (x^2 + y^2)dxdy = 8\pi}$

9. $ \displaystyle{\int\int_D \frac{1}{x^2 + y^2}dxdy = \pi\log(2)}$

10. $ \displaystyle{\int\int_D \log(x^2 + y^2)dxdy = \pi(8\log(2) - 3)}$

13. $ \displaystyle{\left(\sqrt{6}, \frac{3\pi}{4}, 0.6155\right)}$

15. $ \displaystyle{\left(-\sqrt{\frac{3}{2}}, \sqrt{\frac{3}{2}}, -1\right)}$

17. $ \displaystyle{\int\int\int_D (x^2 + y^2 + z^2)dxdydz = \frac{128\pi}{5}}$

19. $ \displaystyle{\int\int\int_D \sin(\sqrt{x^2+y^2+z^2}dxdydz = \frac{\pi}{2}(2\sin(1) + \cos(1) - 2) \approx 0.3506}$

21. (b) $ \dfrac{\pi}{3}(2 - \sqrt{2}) $

23. $ \displaystyle{\int\int\int_D \sqrt{x^2 + y^2}dxdydz = \frac{70\pi}{3}}$

25. $ \dfrac{16}{3} $

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