What will you do?

Algebra Level 2

Evaluate

2 + 5 3 + 2 5 3 . \Large \sqrt[3]{2+\sqrt{5}} + \sqrt[3]{2-\sqrt{5}}.

Give your answer to 3 decimal places.


The answer is 1.000.

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4 solutions

Let A = 2 + 5 3 \color{#E81990}{A}= \sqrt[3]{2+\sqrt{5}} , B = 2 5 3 \color{#EC7300}{B}=\sqrt[3]{2-\sqrt{5}} .
And, S = A + B = 2 + 5 3 + 2 5 3 \Rightarrow\color{#D61F06}{S}=\color{#3D99F6}{A+B}=\sqrt[3]{2+\sqrt{5}}+\sqrt[3]{2-\sqrt{5}}
Now, Cubing both sides.
S 3 = ( A + B ) 3 \Rightarrow \color{#D61F06}{S^3}=\color{#3D99F6}{(A+B)^3}
= A 3 + B 3 + 3 A B ( A + B ) =\color{#3D99F6}{A^3+B^3}+3\color{#20A900}{AB}\color{#3D99F6}{(A+B)}
= A 3 + B 3 + 3 A B S =\color{#3D99F6}{A^3+B^3}+\color{#20A900}{3AB}\color{#D61F06}{S} ...... ( ) (\color{#20A900}{♧}) [ Since , ( A + B ) = S \color{#3D99F6}{(A+B)}=\color{#D61F06}{S} ]
Thus,
A 3 + B 3 = ( 2 + 5 3 ) 3 + ( 2 5 3 ) 3 \Rightarrow \color{#3D99F6}{A^3+B^3}=(\sqrt[3]{2+\sqrt{5}})^{3}+(\sqrt[3]{2-\sqrt{5}})^{3}
= 2 + 5 + 2 5 =2+\sqrt{5}+2-\sqrt{5}
= 4 =\color{#302B94}{4}


3 A B = 3 ( 2 + 5 3 ) ( 2 5 3 ) \Rightarrow \color{#20A900}{3AB}=3(\sqrt[3]{2+\sqrt{5}})(\sqrt[3]{2-\sqrt{5}})
= 3 ( 4 5 3 ) =3(\sqrt[3]{4-5})
= 3 ( 1 3 ) =3(\sqrt[3]{-1})
= 3 × ( 1 ) =3×(-1)
= 3 =\color{#BA33D6}{-3}
Now, putting ( A 3 + B 3 ) = 4 \color{#3D99F6}{(A^3+B^3)}=\color{#302B94}{4} and ( 3 A B ) = 3 \color{#20A900}{(3AB)}=\color{#BA33D6}{-3} in ( ) (\color{#20A900}{♧}) .
S 3 = 4 3 S \Rightarrow \color{#D61F06}{S^3}=\color{#302B94}{4}\color{#BA33D6}{-3}\color{#D61F06}{S}
S 3 + 3 S 4 = 0 \color{#D61F06}{S^3}\color{#BA33D6}{+3}\color{#D61F06}{S}\color{#302B94}{-4}=0
( S 1 ) ( S 2 + S + 4 ) = 0 (\color{#D61F06}{S}-1)(\color{#D61F06}{S^2}+\color{#D61F06}{S}\color{#302B94}{+4})=0
S 1 = 0 \color{#D61F06}{S}-1=0
S = 1 \color{#D61F06}{S}=\color{#624F41}{\boxed{1}}

2 + 5 3 + 2 5 3 = 1 . \Rightarrow \sqrt[3]{2+\sqrt{5}} + \sqrt[3]{2-\sqrt{5}}=\color{#624F41}{\boxed{1}}.


Note : There is a possibilty that ( S 2 + S + 4 = 0 ) (\color{#D61F06}{S^2}+\color{#D61F06}{S}\color{#302B94}{+4}=0) ,but this is not possible because its roots are non-real complex numbers.And the surds given is real.

How did 3(A + B) get transformed into 3AB?

Pieter Breughel - 4 years, 8 months ago

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Thanks! There was a typo.

A Former Brilliant Member - 4 years, 8 months ago
Surya Prakash
Dec 12, 2015

Let y = 2 + 5 3 + 2 5 3 y = \sqrt[3]{2+\sqrt{5}} + \sqrt[3]{2-\sqrt{5}} . Cubing it on both side, we get

y 3 = ( 2 + 5 3 ) 3 + ( 2 5 3 ) 3 + 3 ( 2 + 5 3 ) ( 2 5 3 ) ( 2 + 5 3 + 2 5 3 ) y 3 = ( 2 + 5 ) + ( 2 5 ) + 3 ( ( 2 + 5 ) ( 2 5 ) 3 ) ( y ) y 3 = 4 3 y y 3 + 3 y 4 = 0 y^3 = (\sqrt[3]{2+\sqrt{5}})^3 + (\sqrt[3]{2-\sqrt{5}})^3 + 3( \sqrt[3]{2+\sqrt{5}})( \sqrt[3]{2-\sqrt{5}})(\sqrt[3]{2+\sqrt{5}} + \sqrt[3]{2-\sqrt{5}}) \\ y^3 = (2+\sqrt{5}) + (2-\sqrt{5}) + 3(\sqrt[3]{(2+\sqrt{5})(2-\sqrt{5})})(y) \\ y^3 = 4 - 3y \\ y^3 +3y-4 =0

By trial and error method, we get y = 1 y=1 is a solution of this cubic equation. So, by synthetic division, we get

y 3 + 3 y 4 = ( y 1 ) ( y 2 + y + 4 ) y^3 + 3y -4 = (y-1)(y^2 +y +4)

So, y 3 + 3 y 4 = 0 y = 1 y^3 + 3y -4 = 0 \implies y=1 or y 2 + y + 4 = 0 y^2 + y+ 4 = 0 . But the roots of y 2 + y + 4 = 0 y^2 + y +4 =0 are non-real complex numbers, because the discriminant of the quadratic is negative. The surd given to us is real. So, the only possibility is y = 1 y=1 .

Therefore, 2 + 5 3 + 2 5 3 = 1 \large \sqrt[3]{2+\sqrt{5}} + \sqrt[3]{2-\sqrt{5}} = \boxed{1}

Moderator note:

Interesting approach with writing y y as a polynomial. Is this approach work in the more general case (of roots)?

The same I did. An easy one

Shreyash Rai - 5 years, 6 months ago

same thing, but insted of "trail and error method" use "rational root theorem"

Aareyan Manzoor - 5 years, 6 months ago

How did u factorized the equation(y^3+3y-4) Pls :) ant method or something else ??

Chirayu Bhardwaj - 5 years, 6 months ago

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I've done the same :D

Andrea Virgillito - 4 years, 9 months ago
Chew-Seong Cheong
Dec 15, 2015

We note that: ( 1 ± 5 ) 3 = 1 ± 3 ( 1 2 ) 5 + 3 ( 1 ) ( 5 ) ± 5 5 = 16 ± 8 5 ( 1 ± 5 2 ) 3 = 2 ± 5 \begin{aligned} \text{We note that: } (1\pm\sqrt{5})^3 & = 1\pm3(1^2)\sqrt{5}+3(1)(5) \pm 5\sqrt{5} \\ & = 16 \pm 8\sqrt{5} \\ \Rightarrow \left(\frac{1 \pm \sqrt{5}}{2}\right)^3 & = 2 \pm \sqrt{5} \end{aligned}

Therefore, 2 + 5 3 + 2 5 3 = ( 1 + 5 2 ) 3 3 + ( 1 5 2 ) 3 3 = 1 + 5 2 + 1 5 2 = 1 \begin{aligned} \text{Therefore, } \sqrt[3]{2+\sqrt{5}} + \sqrt[3]{2-\sqrt{5}} & = \sqrt[3]{\left(\frac{1 + \sqrt{5}}{2}\right)^3} + \sqrt[3]{\left(\frac{1 - \sqrt{5}}{2}\right)^3} \\ & = \frac{1 + \sqrt{5}}{2} + \frac{1 - \sqrt{5}}{2} \\ & = \boxed{1} \end{aligned}

How did you thought about ( 1 ± 5 ) 3 (1\pm\sqrt{5})^{3} ?

Aditya Sky - 5 years ago

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Initiation, I thought about what the problem setter was thinking when he set the problem. Problems of this type, we can usually try assuming 2 ± 5 3 = ( a ± b 5 ) 3 3 = a ± b 5 \sqrt[3]{2 \pm \sqrt{5}} = \sqrt[3]{(a \pm b\sqrt{5})^3} = a \pm b\sqrt{5} so that the calculations become easy. So now we have:

2 + 5 = ( a ± b 5 ) 3 = a 3 + 3 a 2 b 5 + 15 a b 2 + 5 b 3 5 \begin{aligned} 2 + \sqrt{5} & = (a \pm b\sqrt{5})^3 \\ & = a^3 + 3a^2b \sqrt{5} + 15ab^2 + 5b^3\sqrt{5} \end{aligned}

Equating coefficients:

{ a 3 + 15 a b 2 = 2 3 a 2 b + 5 b 3 = 1 \begin{cases} a^3 + 15ab^2 = 2 \\ 3a^2b + 5b^3 = 1 \end{cases}

Then you can solve for a a and b b .

I didn't do it that way. I just tried the simplest form ( 1 + 5 ) 3 = 1 + 3 5 + 15 + 5 5 = 16 + 8 5 (1+\sqrt{5})^3 = 1 + 3\sqrt{5} + 15 + 5\sqrt{5} = 16 + 8\sqrt{5} , a = b = 1 2 \implies a = b = \frac{1}{2} .

Chew-Seong Cheong - 5 years ago

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Why did you assume 2 ± 5 3 = ( a ± b 5 ) 3 3 \sqrt[3]{2 \pm \sqrt{5}} = \sqrt[3]{(a \pm b\sqrt{5})^3} and not 2 ± 5 3 = ( a ± b c ) 3 3 \sqrt[3]{2 \pm \sqrt{5}} = \sqrt[3]{(a \pm b\sqrt{c})^3} ?

Aditya Sky - 5 years ago

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@Aditya Sky Then I will have one unknown less. Obviously, 5 \sqrt{5} is present and c = 5 c = \sqrt{5} . It will disappear only if b = 0 b=0 . Since LHS has 5 \sqrt{5} , the RHS must have it or else how could it come about. Note that 5 is a prime.

Chew-Seong Cheong - 5 years ago
Betty BellaItalia
Jun 16, 2017

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