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Problems in Mathematical Analysis.pdf - pwp.net.ipl.pt

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164_Def<strong>in</strong>ite Integrals_\Ch. 5<br />

2. Comput<strong>in</strong>g the volumes of solids from known cross-sections. If S S(x)<br />

is the cross-sectional area cut off by a plane perpendicular to some straight<br />

l<strong>in</strong>e (which we take to be the x-axis) at a po<strong>in</strong>t with abscissa *, then the<br />

volume of the solid is<br />

where *, and x2 are the abscissas of the extreme cross-sections of the solid.<br />

Example 4. Determ<strong>in</strong>e the volume of a wedge cut off a circular cyl<strong>in</strong>der<br />

by a plane pass<strong>in</strong>g through the diameter of the base and <strong>in</strong>cl<strong>in</strong>ed to the base<br />

at an angle a. The radius of the base is R (Fig. 53).<br />

Solution. For the *-axis we take th? diameter of the base along which<br />

the cutt<strong>in</strong>g plane <strong>in</strong>tersects the base, and for the (/-axis we take the diameter<br />

of the base perpendicular to it. The equation of the circumference of the base<br />

is * 2 2 + = j/ R 2 .<br />

The area of the section ABC at a distance x from the orig<strong>in</strong> is<br />

1 1 r/ 2<br />

S(x) = area A ABC = -^ ABBC = -^yy tana =^- tana. Therefore, the sought-<br />

,<br />

for volume of the wedge is<br />

R R<br />

y = 2~ f y 2 tanad*=tana (* (R 1<br />

*)

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