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Algebra Difficulty 3.3 AMC 10/12 Find the answer

The sum 5+2133+52133\sqrt[3] {5+2\sqrt{13}}+\sqrt[3]{5-2\sqrt{13}} equals
(A) 32\text{(A)} \ \frac 32(B) 6534\text{(B)} \ \frac{\sqrt[3]{65}}{4}(C) 1+1362\text{(C)} \ \frac{1+\sqrt[6]{13}}{2}(D) 23\text{(D)}\ \sqrt[3]{2}(E) none of these\text{(E)}\ \text{none of these}

Multiple choice: answer with the letter of the option you want.

Solution

Lets set our original expression equal to xx. So 5+2133+52133=x\sqrt[3] {5+2\sqrt{13}}+\sqrt[3]{5-2\sqrt{13}} = x. Cubing this gives us x3=(5+2133+52133)3=5+213+5213+3(5+213352133)(5+2133+52133)=109xx^3 = \left(\sqrt[3] {5+2\sqrt{13}}+\sqrt[3]{5-2\sqrt{13}}\right)^3 = 5 + 2\sqrt{13} + 5 - 2\sqrt{13} + 3\left(\sqrt[3] {5+2\sqrt{13}}*\sqrt[3]{5-2\sqrt{13}}\right)\left(\sqrt[3] {5+2\sqrt{13}}+\sqrt[3]{5-2\sqrt{13}}\right) = 10 - 9x
So we have x3+9x10=0x^3 + 9x - 10 = 0. We can easily see that 1 is a root of this polynomial. By synthetic division, the new polynomial is x2+x+10x^2 + x + 10, which has no real roots. Thus 5+2133+52133=1\sqrt[3] {5+2\sqrt{13}}+\sqrt[3]{5-2\sqrt{13}} = 1. Since 1 is not A-D, our answer is E\fbox{E}.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.