Topics / Algebra
Exponents, Logarithms & Radicals
Exponent rules, radicals, nested radicals, rationalizing, logarithms (rare)
What you need to know
- Exponent laws: , , , , (). Different bases combine only after rewriting to a common base (, ).
- Radicals: for , and . Rationalize by multiplying by the conjugate .
- Denesting: where and ; verify by squaring.
- Logarithms (rare): , , , .
- To compare and , take a common root: versus .
- is an integer only when is a perfect th power; factor to check.
How AMC 10 tests it
- Problems 1–8: simplify or evaluate , ; the trick is factoring out the smallest power.
- Problems 5–12: solve (match bases) or (isolate, square, check).
- Problems 8–15: "which is largest" among , or the number of digits of .
- Problems 10–18: nested radicals or sums of that rationalize and telescope.
- Logarithms show up only occasionally, usually late in the paper as a disguise for an exponent equation.
Standard approaches
- Express everything in one base and equate exponents.
- In sums of powers, factor out the smallest power: .
- For radical equations, isolate one radical, square, repeat if needed, and test every candidate in the original.
- For nested or infinite radicals, either guess a denested form or set equal to the whole expression and square.
- For size comparisons, take a common power or root, or compare logarithms.
Worked example
Let be a real number with . What is ?
(A) (B) (C) (D) (E)
Let , which is positive. Then
Since , use the sum-of-cubes identity with , , :
The answer is .
Pitfalls
- Writing or .
- Adding exponents across different bases: .
- Keeping an extraneous root after squaring (for example a value that makes a square root negative).
- Forgetting , which loses a negative solution or introduces a sign error.
Traps that recur
- Counting 2014 powers of 2 (including 2^0 or 2^2014) instead of 2^1 through 2^2013, which gives 280. (2014 AMC 10A #25)
- For the fifth root, dividing 312 by 5 and taking the leading digit of the fifth root of 7.77, which ignores that 312 is not a multiple of 5. (2021 AMC Fall 10B #19)
- Guessing that x and y need not be multiples of 7, or splitting 13 unevenly (like 1 and 12) and reporting a larger sum such as 7 * 145. (2024 AMC 10B #13)
- Ignoring the 2-power terms and answering 3^4 = 81 exactly, when the fraction is strictly less than 81 because 2^100 = 16 2^96 is much smaller than 81 2^96. (2018 AMC 10A #14)
Problems, easiest first
How many digits are in the base-ten representation of ?
Let . Which of the following is equal to
What is the value of
What is the value of ?
What is the value of
What is the value of
What is the value of when ?
For what value does ?
What is the value of
What is the value of ?
What is ?
What is the value of
Assume that is a positive real number. Which is equivalent to ?
In the expression , the values of , , , and are , , , and , although not necessarily in that order. What is the maximum possible value of the result?
The ratio is:
The ratio is closest to which of the following numbers?
Find x.
Ted mistakenly wrote as What is the sum of all real numbers for which these two expressions have the same value?
For how many (not necessarily positive) integer values of is the value of an integer?
Suppose that and . Which of the following is equal to for every pair of integers ?
The fraction simplifies to which of the following?
Real numbers and satisfy the equations and . What is ?
Which of the following is equivalent to when ?
What non-zero real value for satisfies ?
How many positive integers satisfy the following condition:
Find the value of that satisfies the equation
Simplify .
Given that where both and are positive integers, find the smallest possible value for .
According to the standard convention for exponentiation, If the order in which the exponentiations are performed is changed, how many other values are possible?
The number is the square of a positive integer . In decimal representation, the sum of the digits of is
Positive integers and satisfy the equation . What is the minimum possible value of ?
What is the greatest integer less than or equal to
Which of the following is equal to ?
Suppose that , , , and . What is ?
Let be the positive integer , a -digit number where each digit is a . Let be the leading digit of the th root of . What is
The number is between and . How many pairs of integers are there such that and