If M = sqrt(4) + sqrt[3]{4} + sqrt[4]{4}, then the value of M is:
GMAT Algebra : (ALG) Questions
If \(\mathrm{M} = \sqrt{4} + \sqrt[3]{4} + \sqrt[4]{4}\), then the value of M is:
- Translate the problem requirements: We need to find the value of \(\mathrm{M} = \sqrt{4} + \sqrt[3]{4} + \sqrt[4]{4}\), which means we're adding the square root of 4, cube root of 4, and fourth root of 4, then comparing this sum to the given ranges in the answer choices.
- Calculate exact and approximate values: Determine the precise value of \(\sqrt{4}\) and reasonable approximations for \(\sqrt[3]{4}\) and \(\sqrt[4]{4}\) that will be sufficient given the answer choice ranges.
- Sum the components strategically: Add the three root values together, using approximations where exact calculation isn't necessary for distinguishing between answer choices.
- Compare against answer choice ranges: Match our calculated sum against the given ranges (less than 3, equal to 3, between 3 and 4, equal to 4, or greater than 4) to identify the correct answer.
Execution of Strategic Approach
1. Translate the problem requirements
Let's start by understanding what we're looking for. We have \(\mathrm{M} = \sqrt{4} + \sqrt[3]{4} + \sqrt[4]{4}\), which means we need to find three different types of roots of the number 4 and add them together.
In everyday language:
- \(\sqrt{4}\) means "what number times itself gives us 4?"
- \(\sqrt[3]{4}\) means "what number times itself three times gives us 4?"
- \(\sqrt[4]{4}\) means "what number times itself four times gives us 4?"
Once we find these three values, we'll add them up to get M, then see which range from our answer choices contains this sum.
Process Skill: TRANSLATE - Converting the root notation into clear mathematical understanding
2. Calculate exact and approximate values
Let's work out each root step by step:
For \(\sqrt{4}\) (square root of 4):
This is the easiest one. What number times itself equals 4? That's \(2 \times 2 = 4\).
So \(\sqrt{4} = 2\) exactly.
For \(\sqrt[3]{4}\) (cube root of 4):
We need a number that when multiplied by itself three times gives us 4.
Let's think about this: \(1^3 = 1\) (too small) and \(2^3 = 8\) (too big).
So \(\sqrt[3]{4}\) must be between 1 and 2, but closer to 1 since 4 is much closer to 1 than to 8.
More precisely, \(\sqrt[3]{4} \approx 1.6\) (since \(1.6 \times 1.6 \times 1.6 \approx 4.1\), which is close to 4)
For \(\sqrt[4]{4}\) (fourth root of 4):
We need a number that when multiplied by itself four times gives us 4.
Again, \(1^4 = 1\) (too small) and \(2^4 = 16\) (way too big).
So \(\sqrt[4]{4}\) must be between 1 and 2, but even closer to 1 than the cube root was.
More precisely, \(\sqrt[4]{4} \approx 1.4\) (since \(1.4^4 \approx 3.8\), which is close to 4)
3. Sum the components strategically
Now let's add up our three values:
\(\mathrm{M} = \sqrt{4} + \sqrt[3]{4} + \sqrt[4]{4}\)
\(\mathrm{M} = 2 + 1.6 + 1.4\)
\(\mathrm{M} = 5.0\)
Even if our approximations are slightly off, let's check the boundaries:
- We know \(\sqrt{4} = 2\) exactly
- We know \(\sqrt[3]{4}\) is definitely greater than 1.5 (since \(1.5^3 = 3.375 < 4\))
- We know \(\sqrt[4]{4}\) is definitely greater than 1.3 (since we can verify this gives us a value less than 4)
So even with conservative estimates: \(\mathrm{M} > 2 + 1.5 + 1.3 = 4.8\)
4. Compare against answer choice ranges
Let's look at our answer choices:
- Less than 3 → Our \(\mathrm{M} \approx 5.0\), so this is definitely false
- Equal to 3 → Our \(\mathrm{M} \approx 5.0\), so this is definitely false
- Between 3 and 4 → Our \(\mathrm{M} \approx 5.0\), so this is false
- Equal to 4 → Our \(\mathrm{M} \approx 5.0\), so this is false
- Greater than 4 → Our \(\mathrm{M} \approx 5.0\), and we showed \(\mathrm{M} > 4.8\) even conservatively, so this is true
Final Answer
The value of M is approximately 5.0, which is greater than 4. Even with the most conservative approximations, we can definitively say \(\mathrm{M} > 4.8\).
Therefore, the answer is E. Greater than 4.
Common Faltering Points
Errors while devising the approach
1. Misinterpreting notation meaning
Students may confuse the different root notations, especially mistaking \(\sqrt[3]{4}\) for \(3\sqrt{4}\) (three times the square root of 4) or \(\sqrt[4]{4}\) for \(4\sqrt{4}\) (four times the square root of 4). This fundamental misunderstanding would lead to calculating \(\mathrm{M} = \sqrt{4} + 3\sqrt{4} + 4\sqrt{4} = 2 + 6 + 8 = 16\), resulting in a completely wrong answer.
2. Believing exact calculation is required
Students might think they need to find the exact decimal values of \(\sqrt[3]{4}\) and \(\sqrt[4]{4}\), becoming overwhelmed by the complexity and missing that strategic approximation and boundary analysis are sufficient to determine which answer range M falls into.
Errors while executing the approach
1. Poor approximation of cube and fourth roots
Students often struggle with estimating \(\sqrt[3]{4}\) and \(\sqrt[4]{4}\) accurately. They might estimate \(\sqrt[3]{4} \approx 1.2\) (too low) or \(\sqrt[4]{4} \approx 1.1\) (too low), leading to \(\mathrm{M} \approx 2 + 1.2 + 1.1 = 4.3\), which might make them incorrectly choose answer choice C (between 3 and 4) instead of E.
2. Computational errors in verification
When checking their approximations, students may make arithmetic mistakes like calculating \(1.6^3\) incorrectly or miscomputing the final sum, leading to an incorrect total that doesn't match the true value of M.
Errors while selecting the answer
1. Boundary confusion
Even with a correct calculation showing \(\mathrm{M} \approx 5.0\), students might hesitate between answer choices D (Equal to 4) and E (Greater than 4) if they're unsure about their approximations, not recognizing that even conservative estimates clearly place M above 4.