Asked by lucy

A frustrum that is cut of has the bottom radius being 63 and the top radius is 21.the height is 12.Find the volume of the frustrum.I 1st used the formula of R/r=H/h and i got the bigger height being 18 while the smaller height being 6cm.1/3*22/7*63*63*18-1/3*22/7*21*21*6.i got the final ans as 74844-2772=72072.am i right.
Answered by oobleck
Since 21 is 1/3 of 63, the frustrum represents 3/4 of the height a complete cone.

So, the missing part is 1/3 the height of the whole cone, which would have a height of 18 if complete. Thus, the missing volume is 1/27 of the whole.

26/27 * 1/3 * π * 63^2 * 18 = 72043

You are correct, using your approximation for π

Answered by lucy
thanku so much
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