What Is A Square Root?
The square root of a number $n$ is the value $r$ for which $r^2 = n$. No whole number squares to $99$, because $9^2 = 81$ is too small and $10^2 = 100$ is too big, so $\sqrt{99}$ falls between $9$ and $10$, very close to $10$.
Simplifying the root means factoring out the largest perfect square inside the number, the same skill that runs through squares and square roots. For $99$, that perfect square is $9$, which turns $\sqrt{99}$ into $3\sqrt{11}$.
Where Does √99 Appear?
$\sqrt{99}$ is the length of the missing leg in a right triangle with hypotenuse $10$ and one leg $1$, because $\sqrt{10^2 - 1^2} = \sqrt{100 - 1} = \sqrt{99}$. It is also the side of a square whose area is $99$ square units, sitting just under a $10 \times 10$ square. Because $99$ is one less than $100$, the value $9.9499$ lands a hair below $10$, which is why $\sqrt{99}$ is a favourite for testing whether a student really simplifies or just rounds.
Quick Reference Table
Every entry is a multiple of $\sqrt{11}$, the exact family $\sqrt{99}$ belongs to.
Number $n$ | $\sqrt{n}$ simplified | Decimal (approx.) |
|---|---|---|
11 | $\sqrt{11}$ | 3.3166 |
44 | $2\sqrt{11}$ | 6.6332 |
99 | $\mathbf{3\sqrt{11}}$ | 9.9499 |
176 | $4\sqrt{11}$ | 13.2665 |
275 | $5\sqrt{11}$ | 16.5831 |
Is The Square Root Of 99 Rational Or Irrational?
$\sqrt{99}$ is irrational. It cannot be written as a fraction $\frac{p}{q}$ of integers, and its decimal neither stops nor repeats.
The prime factorization makes this clear:
$$99 = 3^2 \times 11$$
A number is a perfect square only when every prime appears an even number of times. Here $3$ appears twice but $11$ appears once, which is odd, so $99$ is not a perfect square, and $\sqrt{99}$ is irrational.
How Do You Simplify And Find √99?
What is the square root of 99 in simplest radical form? Prime factorize, then take out the pair.
Start from the factorization.
$$99 = 3^2 \times 11$$
The pair of $3$s comes out of the radical as a single $3$.
$$\sqrt{99} = \sqrt{3^2} \times \sqrt{11}$$
$$\sqrt{99} = 3\sqrt{11}$$
Since $11$ is prime, it has no perfect-square factor, so $3\sqrt{11}$ is fully simplified. For the decimal, use $\sqrt{11} \approx 3.3166$.
$$\sqrt{99} = 3 \times 3.3166$$
$$\sqrt{99} \approx 9.9499$$
You can extend this factor-pulling method to any root in simplifying radical expressions.
Examples Of √99
Example 1
Simplify $\sqrt{99}$ using its largest perfect-square factor.
The largest perfect square dividing $99$ is $9$.
$$\sqrt{99} = \sqrt{9 \times 11}$$
$$\sqrt{99} = 3\sqrt{11}$$
Example 2
A student writes $\sqrt{99} = \sqrt{100} - \sqrt{1} = 9$. Why does that break?
Because $99$ is $100 - 1$, the tempting move is to split the radical across the subtraction:
$$\sqrt{99} = \sqrt{100} - \sqrt{1} = 10 - 1 = 9$$
Check it: $9^2 = 81$, not $99$, so the answer is far too small.
The rule $\sqrt{a - b} = \sqrt{a} - \sqrt{b}$ is false. The correct value is $3\sqrt{11} \approx 9.95$, which is just below $10$, not $9$.
Example 3
Evaluate $\sqrt{99}$ to two decimal places.
$$\sqrt{99} = 3\sqrt{11}$$
$$\sqrt{99} \approx 3 \times 3.3166$$
$$\sqrt{99} \approx 9.95$$
Example 4
Find the missing leg of a right triangle with hypotenuse $10$ and one leg $1$.
$$\text{leg} = \sqrt{10^2 - 1^2}$$
$$\text{leg} = \sqrt{100 - 1}$$
$$\text{leg} = \sqrt{99} = 3\sqrt{11} \approx 9.95$$
Common Mistakes
Mistake 1: Splitting the radical over subtraction
Where it slips in: Because $99 = 100 - 1$ looks convenient.
Don't do this: Writing $\sqrt{99} = \sqrt{100} - \sqrt{1} = 9$.
The correct way: The root of a difference is not the difference of the roots. Simplify by factoring instead: $\sqrt{99} = 3\sqrt{11} \approx 9.95$.
Mistake 2: Rounding 99 up to 100
Where it slips in: Since $\sqrt{99}$ is so close to $10$.
Don't do this: Reporting $\sqrt{99} = 10$.
The correct way: Use $10$ only as an estimate. The exact value $3\sqrt{11} \approx 9.9499$ is a little less than $10$.
Mistake 3: Forgetting to pull out the perfect square
Where it slips in: Leaving the answer as $\sqrt{99}$ when a form is requested.
Don't do this: Calling $\sqrt{99}$ "already simplified."
The correct way: Factor $99 = 9 \times 11$ and pull out $\sqrt{9} = 3$ to get $3\sqrt{11}$. A near-miss like this once mattered in aviation: the 1983 Gimli Glider ran out of fuel because a conversion was left half-done, a reminder to finish the arithmetic.
Conclusion
The square root of 99 is $3\sqrt{11}$, about $9.9499$, and the trick is spotting the perfect square $9$ inside $99$. The value stays irrational because the prime $11$ carries an odd power, and it lands just below $10$ without ever reaching it.
To practise simplifying roots with a teacher, explore Bhanzu's algebra tutor or help with algebra, and see live sessions on math classes online. You can also book a free demo class to run through a handful of roots step by step.
Read More
Square root 1 to 30 — the reference table of the roots you meet most.
Square root tricks — quick ways to estimate and simplify roots by hand.
Square root of 100 — the neighbour that $\sqrt{99}$ sits just below, equal to exactly $10$.
Square root of 20 — another two-step simplification, equal to $2\sqrt{5}$.
Square root 1 to 25 — a compact chart for the smaller roots.
Irrational numbers — why non-perfect-square roots never terminate.
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