Study Guide · Chapter 13
3.5 Third Proportional
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If a : b = b : c, then c is called the third proportional to a and b.
Formula: c = b² / a
Example 24: Find the third proportional to 4 and 8. c = 8²/4 = 64/4 = 16.
Example 25: Find the third proportional to 9 and 12. c = 12²/9 = 144/9 = 16.
Example 25A (additional, decimal values): Find the third proportional to 0.8 and 1.6. c = 1.6²/0.8 = 2.56/0.8 = 3.2.
Example 25B (additional, larger numbers): Find the third proportional to 16 and 24. c = 24²/16 = 576/16 = 36.
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