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9.1 Properties of Similar Figures Proportion—an equation which shows that two ratios are equal. The proportion a = c and a:b = c:d. b d It is read “a is to b as c is to d. Similar Figures—Two figures are similar if these two conditions are true: 1. Corresponding angles are congruent 2. The lengths of corresponding sides are in proportion The symbol ~ means “is similar to” Remember—when you write a proportion, the ratios must compare corresponding lengths. Examples of Similar Figures The trapezoids shown below are similar; they have the same shape but they are different in size. More about Similar Figures Any two corresponding sides in two similar figures have a common ratio called the scale factor. The trapezoids ABCD and EFGH shown above are similar. So, .