1980 AMC 12 Problem 27

Attempt Problem 27 of the 1980 AMC 12 below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 1980 AMC 12 solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

27.

The sum 5+2133+5−2133 \sqrt[3]{5+2\sqrt{13}} +\sqrt[3]{5-2\sqrt{13}} equals
32\frac32
6534\frac{\sqrt[3]{65}}4
1+1362\frac{1+\sqrt[6]{13}}2
23\sqrt[3]2
none of these
Answer: E
Concepts:radicalsum and difference of cubesalgebraic manipulation
Small hint:
Call the two cube roots uu and vv, and compute uvuv
Big hint:
Use (u+v)3=u3+v3+3uv(u+v)(u+v)^3=u^3+v^3+3uv(u+v)
Solution:
Let the two real cube roots be uu and v.v. Then uv=25−523=−3,u3+v3=10. \begin{aligned} uv&=\sqrt[3]{25-52}=-3,\\ u^3+v^3&=10. \end{aligned} If t=u+v,t=u+v, then t3=10−9t,t^3=10-9t, so t3+9t−10=(t−1)(t2+t+10)=0. \begin{gathered} t^3+9t-10\\ =(t-1)(t^2+t+10)\\ =0. \end{gathered} The only real solution is t=1,t=1, which is not listed.
Therefore, the correct answer is E.
LIVE by Po-Shen Loh
Programs

LIVE Classes

Self-Paced Classes

Stars

Scholarship

About Us

Who Is Po-Shen Loh?

Email Contact

Newsletter

Jobs

Become a Star

For a more Thought Full world.

© 2026 Expii, Inc.