What is a Ratio?
Date: 8/17/96 at 20:13:50 From: Lauretta Hughes Subject: Introduction to Ratios I need help understanding ratios. Thanks. Lauretta
Date: 8/30/96 at 16:44:28 From: Doctor Jerry Subject: Re: Introduction to Ratios The term "ratio" can mean many things. It is used, for example, in work with similar triangles. If two triangles are similar, then the ratios of corresponding sides are equal. It is used when quantities are in proportion. If the increase in length of a person's hair is directly proportional to the time since it was last cut, then ratios can be used to calculate various things. If I tell you, for example, that after 4 weeks the increase in length of my hair is about 1/2 inch, then I can ask for the number of weeks required for the increase to be 3.2 inches. Ordinarily, the thought process used is that 4 is to 1/2 as W is to 3.2, where W is the number of weeks required for the increase to be 3.2 inches. This statement is equivalent to the equation 4/(1/2) = W/(3.2). Multiplying by 3.2 gives W = 3.2*(4/(1/2)) = 3.2*(8) = 25.6 weeks. The reason this works can be explained this way: The statement that the increase in length of a person's hair is directly proportional to the time since it was last cut means that I = k*W, where I is the increase in length, W is the number of weeks, and k is a "proportionality constant." So, if I1 and W1 are 1/2 and 4, then I1 = k*W1. So, k = I1/W1. If I2 is 3.2 and W2 is the time required, then I2 = k*W2. But, k = I1/W1. So I2 = (I1/W1)W2. This equation can be rearranged to give W1/W2 = I1/I2, which is the "thought process" mentioned above, that W1 is to W2 as I1 is to I2. -Doctor Jerry, The Math Forum Check out our web site! http://mathforum.org/dr.math/
Search the Dr. Math Library:
Ask Dr. MathTM
© 1994- The Math Forum at NCTM. All rights reserved.