MATH 149 LABORATORY ASSIGNMENT 9 GEOMETRIC APPLICATIONS OF INTEGRATION The definite integral can be used to solve a variety of problems from geometry examples are finding areas between curves lengths of curves volumes and surface area of three dimensional solids and centroids of plane regions In this laboratory we will use MAPLE to calculate the area between curves and the volume of solids of revolution The volume problem will also give us the opportunity to use MAPLE s 3D graphing abilities AREA BETWEEN CURVES In class we showed that if f x g x for all x in a b then the area A of the region enclosed by the curves y f x y g x and the lines x a a nd x b is given by b A f x g x dx a In practice a large part of the problem in using this formula involves determining exactly where f x g x and where g x f x We illustrate with an example EXAMPLE Find the total area of all regions enclosed by the curves f x x sin 2x and g x x3 The first thing to do is to plot the graphs with plots f x x sin 2 x f x x sin 2 x g x x 3 g x x3 c textplot 1 3 4 g 3 1 5 f p plot f g Pi Pi 5 5 display c p Note that the curves intersect at the origin since f 0 g 0 0 Next we try find the X coordinates of the other two points of intersection of the curves solve f x g x x 1 RootOf 8 sin Z 4 Z Z3 2 Apparently there is no nice formula for the solutions but we can still use fsolve to find decimal representations for them X 1 fsolve f x g x x 2 1 X1 1 229835717 X 2 fsolve f x g x x x 1 2 X2 1 229835717 Now we determine the interval or intervals where f x g x and the interval or intervals where g x f x From the graph one can see that g x f x for x in X1 0 and that f x g x for x in 0 X2 Finally we set up the integrals and evaluate them Area Int g x f x x X 1 0 Int f x g x x 0 X 2 Area 0 1 229835717 x sin 2 x x dx 1 229835717 3 x sin 2 x x3 dx 0 value Area 2 145037216 VOLUMES OF SOLIDS OF REVOLUTION A solid generated by revolving a curve y f x about the X axis is called a solid of revolution Examples of such solids are cylinders cones and spheres The command plot3d x f x cos t f x sin t x a b t 0 2 Pi plots the surface generated by revolving y f x a x b about the X axis As an example let f x x2 0 x 1 restart with plots f x x 2 f x x2 plot3d x f x cos t f x sin t x 0 1 t 0 2 Pi Click on the figure and use the toolbar to rotate the solid and change the appearance of the coordinate system You can also rotate the solid by moving the cursor while holding down the left mouse button You may plot several graphs on the same coordinate system For example let s intersect the above graph with the X Y plane A B plot3d x y 0 x 0 1 5 y 1 5 1 5 display3d A B The formula for the volume V of the solid resulting from rotating y f x a x b about the X axis is b V f x 2 dx a Thus in the above example 1 2 2 V x dx 0 and therefore V int Pi x 4 x 0 1 evalf V V 5 0 6283185308
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