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14.9 Assess Your Understanding

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Concepts and Vocabulary

  1. For a change of variables from x and y to u and v given by x=g1(u,v) and y=g2(u,v), the Jacobian is the determinant _____.

|xuxvyuyv|

  1. True or False When introducing a Jacobian into an integral because of a change of variables, the absolute value of the Jacobian is used.

True

Skill Building

In Problems 3–10, find the Jacobian (x,y)(u,v) for each change of variables from (x,y) to (u,v).

  1. x=u+v,y=uv

(x,y)(u,v)=2

  1. x=u+5,y=v7

  1. x=4uv,y=3u+12v

(x,y)(u,v)=5

  1. x=u2v2,y=u2+v2

  1. x=uv,y=u+v

(x,y)(u,v)=u+vv2

  1. x=eucosv,y=eusinv

  1. x=veu,y=uev

(x,y)(u,v)=euv(1+uv)

  1. x=ucosv,y=vsinu

In Problems 11–16, find the Jacobian (x,y,z)(u,v,w) for each change of variables from (x,y,z) to (u,v,w).

  1. x=u+v+w,y=u+vw,z=uvw

(x,y,z)(u,v,w)=4

  1. x=2u+v+w,y=u+vw,z=uv

  1. x=u+v,y=vw,z=uw

(x,y,z)(u,v,w)=2

  1. x=u+v+w,y=u+v,z=w

  1. x=u,y=v2,z=w3

(x,y,z)(u,v,w)=6vw2

  1. x=3(u+v),y=uw,z=u2w2

  1. Area Find the area of the region R enclosed by y=41x29 and y=0, using the change of variables u=x3 and v=y2.

A=6π square units

  1. Area Find the area of the region R enclosed by xy=1, xy=3, y=x, and y=3x using the change of variables u=x and v=xy. See the figure.

In Problems 19–22:

  1. (a) Graph the region R in the xy-plane.
  2. (b) Graph the region R# in the uv-plane.
  3. (c) Find the Jacobian (x,y)(u,v).
  4. (d) Change the variables and find the integral.

  1. , where R is the region enclosed by 2x-y=0, 2x-y=4, x+y=0, and x+y=3. Use the change of variables u=2x-y and v=x+y.

  1. (a)
  2. (b)
  3. (c) \dfrac{\partial (x,y)}{\partial (u,v)} = \dfrac13
  4. (d) \dfrac{14}3
  1. \iint\limits_{\kern-3ptR}(x^{2}+y^{2})\,{\it dA}, where R is the region enclosed by 2x-y=1, 2x-y=3, x+y=1, and x+y=2. Use the change of variables u=2x-y and v=x+y.

  1. \iint\limits_{\kern-3ptR}xy\,{\it dA}, where R is the triangular region whose vertices are (-1,1), (1,1), and (0,0). Use the change of variables u=x+y and v=x-y.

  1. (a)
  2. (b)
  3. (c) \dfrac{\partial (x,y)}{\partial (u,v)} = -\dfrac12
  4. (d) 0
  1. \iint\limits_{\kern-3ptR}(x+ y) sin(x-y) {\it dA}, where R is the triangular region whose vertices are (-1,1), (1,1), and (0,0); use the change of variables u=x+y and v=x-y.

In Problems 23–26, find each integral.

  1. \iint\limits_{\kern-3ptR}(3x-2y)\,{\it dA}, where R is the region enclosed by the ellipse 9x^{2}+16y^{2}=144. Change the variables using u=\dfrac{x}{4} and v=\dfrac{y}{3}.

0

  1. \iint\limits_{\kern-3ptR}(x^{2}-y), {\it dA}, where R is the region enclosed by the ellipse 9x^{2}+16y^{2}=144. Change the variables using u=\dfrac{x}{4} and v=\dfrac{y}{3}.

  1. \iiint\limits_{\kern-3ptE}(x+1)\,{\it dV}, where E is the solid enclosed by the ellipsoid \dfrac{x^{2}}{4}+\dfrac{y^{2}}{4}+ \dfrac{z^{2}}{9}=1. Change the variables using u=\dfrac{x}{2}, v=\dfrac{y }{2}, and w=\dfrac{z}{3}.

16 \pi

  1. \iiint\limits_{\kern-3ptE}(x+2y)\,{\it dV}, where E is the region enclosed by the ellipse \dfrac{x^{2}}{9}+\dfrac{y^{2}}{4}+z^{2}=1. Change the variables using u= \dfrac{x}{3}, v=\dfrac{y}{2}, and w=z.

Applications and Extensions

In Problems 27–30, for each integral:

  1. (a) Graph the region R.
  2. (b) Choose a change of variables from (x,y) to (u,v). Graph the new region in the uv-plane.
  3. (c) Write each integral using the new variables.
  4. (d) Find each integral.

  1. \iint\limits_{\kern-3ptR}x^{2}y\,{\it dA}, where R is the region enclosed by the lines 2x-3y=0, 2x-3y=1, x+2y=0, and x+2y=3

  1. (a)
  2. (b)
  3. (c) \int_0^1 \int_0^3 \left(\dfrac37 v+ \dfrac27 u\right)^2 \left(\dfrac27 v - \dfrac17 u \right)\dfrac17 \, dv \, du
  4. (d) \dfrac{423}{2401}
  1. \iint\limits_{\kern-3ptR}x\,{\it dA}, where R is the region enclosed by the ellipse \dfrac{x^{2}}{9}+\dfrac{y^{2}}{16}=1

  1. \iint\limits_{\kern-3ptR}x\cos (xy) \,{\it dA}, where R is the region enclosed by xy=1, xy=3, x=1, and x=3

  1. (a)
  2. (b)
  3. (c) \int_1^3 \int_1^3 \cos(v) \, dv \, du
  4. (d) 2 (\sin 3 - \sin 1)
  1. \iint\limits_{\kern-3ptR}ye^{xy}\,{\it dA}, where R is the region enclosed by xy=1, xy=3, x=1, and x=3

  1. Area Use a change of variables to find the area of the region R enclosed by the lines y=3x, y-3x=2, 2y+x=0, and y=-\dfrac{1}{2} x+5.

A = \dfrac{20}7 square units

968

  1. Area Use a change of variables to find the area of the region R enclosed by the lines x+y=1, x+y=-3, y=2x, and y=2x+4.

  1. Volume Use a change of variables to find the volume V of the solid E enclosed by the planes x+2y=0, x+2y=3, y-z=0, y-z=2, z=0, and z=6.

V = 36 cubic units

  1. Volume Use a change of variables to find the volume V of the solid E enclosed by the planes x-y=0, x-y=3, z-2x=0, z-2x=4, z=1, and z=5.

  1. Volume Find the volume of the solid E enclosed by the paraboloid z=9- x^{2}-9y^{2} and the plane z=1, given the change of variables x=3u\cos v, y=u\sin v, and z=w. See the figure.

V = \dfrac{21\pi}2 cubic units

  1. Volume Find the volume of the solid E enclosed by the ellipsoid \dfrac{x^{2}}{4}+\dfrac{y^{2}}{9}+z^{2}=1 above the xy-plane, given the change of variables u=\dfrac{x}{2}, v=\dfrac{y}{3}, and w=z.

  1. Show that in changing from rectangular coordinates (x, y, z) to spherical coordinates (\rho, \theta, \phi), the triple integral of f over E becomes \begin{eqnarray*} &&\iiint\limits_{\kern-18ptE}f(x, y, z)\,{\it dV}\\[4pt] &&\quad=\iiint\limits_{\kern-18ptE^{\#}}f(\rho \sin \phi \cos \theta , \rho \sin \phi \sin \theta , \rho \cos \phi )\rho ^{2}\sin \phi \,d\rho \,d\theta \,d\phi \end{eqnarray*}

See the Student Solutions Manual.

Challenge Problems

  1. A region R is enclosed by the graphs of xy=2, xy=5, y= \sqrt{x}, and y=3\sqrt{x}.

    1. (a) Graph the region R.
    2. (b) Use a change of variables that transforms R into a rectangular region R^{\#}.
    3. (c) Find the area of R.