= (The product of the = the product of the ad bc mean terms extreme Worked examples 1. Find the fourth proportional to 5 ; 8 ; Let be the fourth proportional x i.e.: = Then: 5x = 8() 5x = 100 = 20 x 2. Find the third proportional to 5 and 8. Let be the third proportional x i.e: = 5x = 8(8) 5x = 64 = = 12 x Activity 2 Calculate the fourth proportional to Calculate the mean proportional 3, 5 and 8. between 14 and 56 Calculate the third proportional to 8 Calculate the mean proportional and 14. between 30 and 48. Solve for if 28 x = 7 3 Solve for if 18 : = : 8 : : x x x x Theorem 1: A line drawn parallel to one side of a triangle divides the other two sides proportionally (line one side of )
ad = bc (The product of the mean terms = the product of the extreme Worked examples 1. Find the fourth proportional to 5 ; 8 ; Let x be the fourth proportional i.e.: = Then: 5x = 8() 5x = 100 x = 20 2. Find the third proportional to 5 and 8. Let x be the third proportional i.e: = 5x = 8(8) 5x = 64 x = = 12 Activity 2 Calculate the fourth proportional to 3, 5 and 8. Calculate the mean proportional between 14 and 56 Calculate the third proportional to 8 and 14. Calculate the mean proportional between 30 and 48. Solve for x if 28 : x = 7 : 3 Solve for x if 18 : x = x : 8 Theorem 1: A line drawn parallel to one side of a triangle divides the other two sides proportionally (line one side of )
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