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Length of DE= 15 cm
Length of AB = 6 cm
Two triangles are similar if they have the same ratio of corresponding sides (among another rule which is not needed here). This means that the length of corresponding sides are proportional. We can see in triangle CDE that side CE is 24 cm. This corresponds to side AC in triangle ABC which is 8 cm. Using these two measurements we can see that the larger triangle is 3 times the size of the smaller triangle. Using equal proportions we can say that line segment BC which is 5 cm corresponds to line segment DE. Line segment DE is 3 times larger, so length of DE is 5x3=15 cm. In triangle CDE line segment CD is given a length of 18 cm. Using the proportion that we now know, we can say that in triangle ABC that line segment AB is 3 times smaller than line segment CD. 18/3= 6 cm. Line segment AB is 6 cm.