The phrase "theory of nines" is ambiguous, referring to two distinct concepts, both known as "rule of nines": one in mathematics and one in medicine.
Mathematical Rule of Nines: Divisibility by 9
The mathematical rule of nines is a straightforward method to check if a number is divisible by nine.
How it Works
This rule is based on the property that a number is divisible by 9 if the sum of its digits is also divisible by 9.
- Add the Digits: Sum all the individual digits of the number.
- Check Divisibility: If the sum obtained in step 1 is divisible by 9, then the original number is also divisible by 9.
- Repeat if Necessary: If the sum is still a multi-digit number, you can repeat the process of adding its digits until you arrive at a single-digit number. This final number will be divisible by 9 if and only if the original number was.
Examples
| Original Number | Sum of Digits | Divisible by 9? |
|---|---|---|
| 81 | 8 + 1 = 9 | Yes |
| 126 | 1 + 2 + 6 = 9 | Yes |
| 345 | 3 + 4 + 5 = 12 | No |
| 999 | 9 + 9 + 9 = 27 | Yes |
| 12345 | 1 + 2 + 3 + 4 + 5 = 15 | No |
| 873 | 8 + 7 + 3 = 18 | Yes |
- In the last example, since 18 is divisible by 9, then 873 is divisible by 9.
- The divisibility by 9 rule simplifies checking for large numbers.
Wallace Rule of Nines: Burn Assessment
The Wallace rule of nines is a medical tool, more specifically used for quickly estimating the total body surface area (TBSA) affected by burns in burn patients.
How it Works
This rule assigns percentage values to different body parts, each either 9% or a multiple of 9% for adults. This is crucial for medical professionals determining the severity of burn cases and planning appropriate treatment.
- Head and Neck: 9%
- Each Arm: 9%
- Each Leg: 18%
- Anterior Trunk: 18%
- Posterior Trunk: 18%
- Perineum: 1%
Important Notes
- This rule is mainly for adults, as proportions differ significantly in children. For example, an infant's head accounts for a larger proportion of their body area than an adult's. Pediatric versions of the rule exist to account for this difference.
- The Wallace rule of nines provides a quick approximation; more precise assessment tools may be necessary for more severe cases.
- This rule is primarily used when assessing second and third-degree burns. Superficial first-degree burns are usually not calculated.
- The Wallace rule of nines is used as a guideline for estimating fluid resuscitation, the risk of complications, and appropriate medical interventions.
In conclusion, the “theory of nines” doesn’t refer to a singular theory, but rather to two different concepts, both known as the "rule of nines." The mathematical one involves divisibility, while the medical one relates to burn assessments.
