AMC 10 Step by Step

Topics / Algebra

Exponents, Logarithms & Radicals

Exponent rules, radicals, nested radicals, rationalizing, logarithms (rare)

36
primary-topic problems (2.8% of all)
21
more as a secondary topic
Where it appears
25
P1-10
7
P11-15
3
P16-20
1
P21-25

What you need to know

  • Exponent laws: aman=am+na^ma^n=a^{m+n}, (am)n=amn(a^m)^n=a^{mn}, an=1ana^{-n}=\dfrac1{a^n}, a1/n=ana^{1/n}=\sqrt[n]a, a0=1a^0=1 (a0a\ne0). Different bases combine only after rewriting to a common base (4x=22x4^x=2^{2x}, 27=3327=3^3).
  • Radicals: ab=ab\sqrt a\sqrt b=\sqrt{ab} for a,b0a,b\ge0, and a2=a\sqrt{a^2}=|a|. Rationalize 1a±b\dfrac1{\sqrt a\pm\sqrt b} by multiplying by the conjugate ab\sqrt a\mp\sqrt b.
  • Denesting: a±2b=m±n\sqrt{a\pm2\sqrt b}=\sqrt m\pm\sqrt n where m+n=am+n=a and mn=bmn=b; verify by squaring.
  • Logarithms (rare): logbx=y    by=x\log_b x=y\iff b^y=x, log(xy)=logx+logy\log(xy)=\log x+\log y, logxk=klogx\log x^k=k\log x, logbx=logxlogb\log_b x=\dfrac{\log x}{\log b}.
  • To compare 23002^{300} and 32003^{200}, take a common root: 81008^{100} versus 91009^{100}.
  • Nn\sqrt[n]{N} is an integer only when NN is a perfect nnth power; factor NN to check.

How AMC 10 tests it

  • Problems 1–8: simplify 22020+2202222021\dfrac{2^{2020}+2^{2022}}{2^{2021}} or evaluate 82/38^{2/3}, (2)6(\sqrt2)^6; the trick is factoring out the smallest power.
  • Problems 5–12: solve 2x+1=8x22^{x+1}=8^{x-2} (match bases) or x+5x=1\sqrt{x+5}-\sqrt x=1 (isolate, square, check).
  • Problems 8–15: "which is largest" among 240,330,5202^{40},3^{30},5^{20}, or the number of digits of 2k5m2^{k}5^{m}.
  • Problems 10–18: nested radicals 6+6+\sqrt{6+\sqrt{6+\cdots}} or sums of 1k+k+1\frac1{\sqrt k+\sqrt{k+1}} that rationalize and telescope.
  • Logarithms show up only occasionally, usually late in the paper as a disguise for an exponent equation.

Standard approaches

  1. Express everything in one base and equate exponents.
  2. In sums of powers, factor out the smallest power: 2n+2n+2=2n(1+4)2^n+2^{n+2}=2^n(1+4).
  3. For radical equations, isolate one radical, square, repeat if needed, and test every candidate in the original.
  4. For nested or infinite radicals, either guess a denested form m±n\sqrt m\pm\sqrt n or set xx equal to the whole expression and square.
  5. For size comparisons, take a common power or root, or compare logarithms.

Worked example

Let xx be a real number with 4x+4x=344^x+4^{-x}=34. What is 8x+8x8^x+8^{-x}?

(A) 102102 (B) 162162 (C) 198198 (D) 204204 (E) 216216

Let t=2x+2xt=2^x+2^{-x}, which is positive. Then
t2=4x+2+4x=36,t=6. t^2=4^x+2+4^{-x}=36,\qquad t=6 .
Since 8x=(2x)38^x=(2^x)^3, use the sum-of-cubes identity u3+v3=(u+v)33uv(u+v)u^3+v^3=(u+v)^3-3uv(u+v) with u=2xu=2^x, v=2xv=2^{-x}, uv=1uv=1:
8x+8x=t33t=21618=198. 8^x+8^{-x}=t^3-3t=216-18=198 .
The answer is (C) 198\boxed{\textbf{(C)}\ 198}.

Pitfalls

  • Writing a+b=a+b\sqrt{a+b}=\sqrt a+\sqrt b or (a+b)2=a2+b2(a+b)^2=a^2+b^2.
  • Adding exponents across different bases: 2332652^3\cdot3^2\ne6^5.
  • Keeping an extraneous root after squaring (for example a value that makes a square root negative).
  • Forgetting x2=x\sqrt{x^2}=|x|, 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

2023 AMC 10A · #5Exponents, Logarithms & Radicals

How many digits are in the base-ten representation of 855101558^5 \cdot 5^{10} \cdot 15^5 ?

2021 AMC Fall 10B · #5Exponents, Logarithms & Radicals

Let n=82022n=8^{2022} . Which of the following is equal to n4?\frac{n}{4}?

2021 AMC 10A · #1Exponents, Logarithms & Radicals

What is the value of (222)(323)+(424)?(2^2-2)-(3^2-3)+(4^2-4)?

2021 AMC 10B · #2Exponents, Logarithms & Radicals

What is the value of (323)2+(3+23)2\sqrt{\left(3-2\sqrt{3}\right)^2}+\sqrt{\left(3+2\sqrt{3}\right)^2} ?

2019 AMC 10A · #1Exponents, Logarithms & Radicals

What is the value of 2(0(19))+((20)1)9?2^{\left(0^{\left(1^9\right)}\right)}+\left(\left(2^0\right)^1\right)^9?

2018 AMC 10A · #1Exponents, Logarithms & Radicals

What is the value of (((2+1)1+1)1+1)1+1?\left(\left((2+1)^{-1}+1\right)^{-1}+1\right)^{-1}+1?

2016 AMC 10B · #1Exponents, Logarithms & Radicals

What is the value of 2a1+a12a\frac{2a^{-1}+\frac{a^{-1}}{2}}{a} when a=12a= \tfrac{1}{2} ?

2016 AMC 10A · #2Exponents, Logarithms & Radicals

For what value xx does 10x1002x=1000510^{x}\cdot 100^{2x}=1000^{5} ?

2015 AMC 10A · #1Exponents, Logarithms & Radicals

What is the value of (201+52+0)1×5?(2^0-1+5^2+0)^{-1}\times5?

2015 AMC 10B · #1Exponents, Logarithms & Radicals

What is the value of 2(2)22-(-2)^{-2} ?

2014 AMC 10B · #2Exponents, Logarithms & Radicals

What is 23+2323+23\frac{2^3 + 2^3}{2^{-3} + 2^{-3}} ?

2013 AMC 10A · #8Exponents, Logarithms & Radicals

What is the value of 22014+220122201422012 ?\frac{2^{2014}+2^{2012}}{2^{2014}-2^{2012}} ?

2008 AMC 10B · #3Exponents, Logarithms & Radicals

Assume that xx is a positive real number. Which is equivalent to xx3\sqrt[3]{x\sqrt{x}} ?

2004 AMC 10B · #5Exponents, Logarithms & Radicals

In the expression cabdc\cdot a^b-d , the values of aa , bb , cc , and dd are 00 , 11 , 22 , and 33 , although not necessarily in that order. What is the maximum possible value of the result?

2002 AMC 10B · #1Exponents, Logarithms & Radicals

The ratio 220013200362002\frac{2^{2001}\cdot3^{2003}}{6^{2002}} is:

2002 AMC 10A · #1Exponents, Logarithms & Radicals

The ratio 102000+102002102001+102001\frac{10^{2000}+10^{2002}}{10^{2001}+10^{2001}} is closest to which of the following numbers?

2000 AMC 10 · #2Exponents, Logarithms & Radicals

2000(20002000)=x2000(2000^{2000}) = x Find x.

2022 AMC 10A · #11Exponents, Logarithms & Radicals

Ted mistakenly wrote 2m140962^m\cdot\sqrt{\frac{1}{4096}} as 214096m.2\cdot\sqrt[m]{\frac{1}{4096}}. What is the sum of all real numbers mm for which these two expressions have the same value?

2018 AMC 10A · #7Exponents, Logarithms & Radicals

For how many (not necessarily positive) integer values of nn is the value of 4000(25)n4000\cdot \left(\tfrac{2}{5}\right)^n an integer?

2009 AMC 10A · #13Exponents, Logarithms & Radicals

Suppose that P=2mP = 2^m and Q=3nQ = 3^n . Which of the following is equal to 12mn12^{mn} for every pair of integers (m,n)(m,n) ?

2008 AMC 10A · #7Exponents, Logarithms & Radicals

The fraction (32008)2(32006)2(32007)2(32005)2\frac{\left(3^{2008}\right)^2-\left(3^{2006}\right)^2}{\left(3^{2007}\right)^2-\left(3^{2005}\right)^2} simplifies to which of the following?

2007 AMC 10A · #9Exponents, Logarithms & Radicals

Real numbers aa and bb satisfy the equations 3a=81b+23^{a} = 81^{b + 2} and 125b=5a3125^{b} = 5^{a - 3} . What is abab ?

2006 AMC 10B · #7Exponents, Logarithms & Radicals

Which of the following is equivalent to x1x1x\sqrt{\frac{x}{1-\frac{x-1}{x}}} when x<0x < 0 ?

2006 AMC 10A · #6Exponents, Logarithms & Radicals

What non-zero real value for xx satisfies (7x)14=(14x)7(7x)^{14}=(14x)^7 ?

2005 AMC 10A · #13Exponents, Logarithms & Radicals

How many positive integers nn satisfy the following condition: (130n)50>n100>2200 ?\left(130n\right)^{50} > n^{100} > 2^{200} \ \text{?}

2003 AMC 10B · #9Exponents, Logarithms & Radicals

Find the value of xx that satisfies the equation 252=548/x526/x2517/x.25^{-2} = \frac{5^{48/x}}{5^{26/x} \cdot 25^{17/x}}.

2003 AMC 10A · #9Exponents, Logarithms & Radicals

Simplify xxxx333\sqrt[3]{x\sqrt[3]{x\sqrt[3]{x\sqrt{x}}}} .

2003 AMC 10B · #14Exponents, Logarithms & Radicals

Given that 3852=ab,3^8\cdot5^2=a^b, where both aa and bb are positive integers, find the smallest possible value for a+ba+b .

2002 AMC 10A · #3Exponents, Logarithms & Radicals

According to the standard convention for exponentiation, 2222=2(2(22))=216=65536.2^{2^{2^{2}}} = 2^{(2^{(2^2)})} = 2^{16} = 65536. If the order in which the exponentiations are performed is changed, how many other values are possible?

2002 AMC 10B · #14Exponents, Logarithms & Radicals

The number 2564642525^{64}\cdot 64^{25} is the square of a positive integer NN . In decimal representation, the sum of the digits of NN is

2024 AMC 10B · #13Exponents, Logarithms & Radicals

Positive integers xx and yy satisfy the equation x+y=1183\sqrt{x} + \sqrt{y} = \sqrt{1183} . What is the minimum possible value of x+yx+y ?

2018 AMC 10A · #14Exponents, Logarithms & Radicals

What is the greatest integer less than or equal to 3100+2100396+296?\frac{3^{100}+2^{100}}{3^{96}+2^{96}}?

2011 AMC 10A · #16Exponents, Logarithms & Radicals

Which of the following is equal to 962+9+62\sqrt{9-6\sqrt{2}}+\sqrt{9+6\sqrt{2}} ?

2005 AMC 10B · #17Exponents, Logarithms & Radicals

Suppose that 4a=54^a = 5 , 5b=65^b = 6 , 6c=76^c = 7 , and 7d=87^d = 8 . What is abcda \cdot b\cdot c \cdot d ?

2021 AMC Fall 10B · #19Exponents, Logarithms & Radicals

Let NN be the positive integer 77777777777\ldots777 , a 313313 -digit number where each digit is a 77 . Let f(r)f(r) be the leading digit of the rr{ } th root of NN . What is f(2)+f(3)+f(4)+f(5)+f(6)?f(2) + f(3) + f(4) + f(5)+ f(6)?

2014 AMC 10A · #25Exponents, Logarithms & Radicals

The number 58675^{867} is between 220132^{2013} and 220142^{2014} . How many pairs of integers (m,n)(m,n) are there such that 1m20121\leq m\leq 2012 and 5n<2m<2m+2<5n+1?5^n<2^m<2^{m+2}<5^{n+1}?