Cube root of 1

Algebra Level 2

How many roots does cube root of 1 have?

3 2 5 1

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1 solution

Tunk-Fey Ariawan
Mar 1, 2014

The cube root of 1 can be obtained by solving z 3 = 1 z^3=1 . Therefore z 3 = 1 z 3 1 = 0 ( z 1 ) ( z 2 + z + 1 ) = 0 \begin{aligned} z^3&=1\\ z^3-1&=0\\ (z-1)(z^2 + z + 1) &= 0 \end{aligned} Clearly that solution of the last equation has 3 roots. One real root from the solution of z 1 = 0 z-1=0 and two complex roots from the solution of z 2 + z + 1 = 0 z^2 + z + 1=0 . Thus, the cube roots of 1 are: 1 3 = { 1 , 1 2 + 1 2 3 i , 1 2 1 2 3 i . \sqrt[3]{1} =\left\{ \begin{array}{l l} \quad 1,\\ \\ -\frac{1}{2}+\frac{1}{2}\sqrt{3}\;i,\\ \\ -\frac{1}{2}-\frac{1}{2}\sqrt{3}\;i. \end{array} \right. # Q . E . D . # \text{\# }\mathbb{Q.E.D.}\text{ \#}

Clear solution.

Victor Loh - 7 years, 2 months ago

Note that the fundamental theorem of algebra states that every non-zero, single-variable, degree n polynomial with complex coefficients has, counted with multiplicity, exactly n roots,hence the answer.

King Zhang Zizhong - 7 years, 2 months ago

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You are wrong. Its " at most " n roots and not exactly n roots.

Nihar Mahajan - 5 years, 7 months ago

the ques. was that how many roots of cube root of one are there ? So,we could write it like this: x = \­(\sqrt [ 3 ]{ 1 } ) . BUt after this we cannot change the degree of the following equation . Please reply me.

Tushar Vishi - 7 years, 1 month ago

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