Surface Area of a Right CylinderDate: 08/21/2001 at 21:49:10 From: Ramonda Dugas Subject: Surface area of a right cylinder in centimeters but wanting answer in meters I'm a home school mom trying to learn geometry before I teach it to my kids. The problem in my book asked me to find the surface area of a right cylinder in centimeters with the dimensions given in meters. The radius of the end is 10 meters and the length of the cylinder is 15 meters. I know my formula is 2(area end)+ (CxL). This is how they achieved the answer: 2(3.14)(10^2)+ 20(3.14)(15) = 628+942=1570(100)(100)= 15,700,000cm^2 What I want to know is why you can't multiply the 10 and 15 by 100 before you start working the problem to get the correct answer. It does not come out to the same answer doing it that way. Ramonda Dugas Date: 08/21/2001 at 23:27:12 From: Doctor Peterson Subject: Re: Surface area of a right cylinder in centimeters but wanting answer in meters Hi, Ramonda. There are three ways to do this, all of which are instructive. You should get the same answer using either of your two methods. First, you can convert to the new units at the start as you suggest; you will have a radius of 1000 cm and a length of 1500 cm, giving A = 2 pi (1000)^2 + pi 2(1000)(1500) = 6,280,000 + 9,420,000 = 15,700,000 cm^2 Second, you can find the area and then convert the square units at the end: A = 2 pi (10)^2 + pi 2(10)(15) = 628 + 942 = 1570 m^2 1570 m^2 (100)(100) = 15,700,000 cm^2 Third, you can do it all at once, including the conversion factors in your formula: A = 2 pi (10 m)^2 + pi 2(10 m)(15 m) = 2 pi (10 m * 100 cm/m)^2 + pi 2(10 m * 100 cm/m)(15 m * 100 cm/m) = 2 pi (1000 cm)^2 + pi 2(1000 cm)(1500 cm) = 6,280,000 cm^2 + 9,420,000 cm^2 = 15,700,000 cm^2 Notice that in each term we are multiplying by 100 twice, once for each dimension, which explains why we convert from square meters to square centimeters by multiplying by the square of 100. This method is just the same as either of the others, with labels included to clarify what we are doing. I suspect you may have forgotten to multiply the 10 by 100 before squaring it, and only multiplied the first term by 100 rather than 10000, or something like that. But your suggested method is in fact good, and is what I recommend to beginners who haven't yet caught on to the conversion of square units. - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/ Date: 08/22/2001 at 10:42:55 From: Ramonda Dugas Subject: Re: Surface area of a right cylinder in centimeters but wanting answer in meters Thank you. I thought I should be able to do that at the beginning but somehow I kept coming up with a higher number. Must have punched in too many zeros in my multiplications. Ramonda |
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