Study Guide · Chapter 14
3.6 Fourth Proportional
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If a : b = c : d, then d is called the fourth proportional to a, b, c.
Formula: d = (b × c) / a
Example 26: Find the fourth proportional to 2, 3, and 4. d = (3 × 4)/2 = 6.
Example 27: Find the fourth proportional to 5, 8, 15. d = (8 × 15)/5 = 120/5 = 24.
Example 27A (additional): Find the fourth proportional to 7, 21, and 12. d = (21 × 12)/7 = 252/7 = 36.
Example 27B (additional): Find the fourth proportional to 9, 12, and 15. d = (12 × 15)/9 = 180/9 = 20.
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