Quirky equation III

Algebra Level 3

Solve for x x : x + x + 1 2 + x + 1 4 = 1024 x+\sqrt{x+\dfrac{1}{2}+\sqrt{x+\dfrac{1}{4}}}=1024


The answer is 992.

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3 solutions

x + x + 1 2 + x + 1 4 = 1024 x + \sqrt{x + \dfrac{1}{2} + \sqrt{x+\dfrac{1}{4} } } =1024

x + x + 1 4 + 1 4 + 2 × 1 2 × x + 1 4 = 2 10 x + \sqrt{ x + \dfrac{1}{4} + \dfrac{1}{4} + 2\times \dfrac{1}{2} \times \sqrt{x + \dfrac{1}{4} } } = 2^{10}

x + ( x + 1 4 + 1 2 ) 2 = 2 10 x + \sqrt{ \left ( \sqrt{x + \dfrac{1}{4} } + \dfrac{1}{2} \right)^2 } = 2^{10}

x + 1 2 + x + 1 4 = 2 10 x + \dfrac{1}{2} + \sqrt{ x + \dfrac{1}{4} } = 2^{10}

Splitting the terms the same way again, ( x + 1 4 + 1 2 ) 2 = 2 10 \left (\sqrt{ x + \dfrac{1}{4} } + \dfrac{1}{2} \right)^{2} = 2^{10}

x + 1 4 = 32 1 2 \sqrt{ x + \dfrac{1}{4}} = 32 - \dfrac{1}{2}

Squaring,

x + 1 4 = 1024 2 × 1 2 × 32 + 1 4 x + \dfrac{1}{4} = 1024 -2 \times \dfrac{1}{2} \times 32 + \dfrac{1}{4}

x = 1024 32 = 992 x = 1024 - 32 = 992

x + x + 1 2 + x + 1 4 = 1024 x+ \sqrt{x+ \dfrac{1}{2}+ \sqrt{x+\dfrac{1}{4}}}=1024

Making a variable change we get the following :

x + 1 4 = y \sqrt{x+\dfrac{1}{4}}=y

Squaring both sides and adding 1 4 \dfrac{1}{4} we get:

x + 1 2 = y 2 + 1 4 x+ \dfrac{1}{2}=y^2+\dfrac{1}{4}

From this we get:

x = y 2 1 4 x=y^2- \dfrac{1}{4}

Substituting this three equations into our original equation we get:

y 2 1 4 + y 2 + 1 4 + y = 1024 y^2- \dfrac{1}{4}+ \sqrt{y^2+ \dfrac{1}{4}+ y}=1024

We factorize inside the root, simplify and finally solve the equation:

y 2 1 4 + ( y + 1 2 ) 2 = 1024 y^2- \dfrac{1}{4}+ \sqrt{(y+ \dfrac{1}{2})^2}=1024

y 2 1 4 + y + 1 2 = 1024 y^2- \dfrac{1}{4}+ y+ \dfrac{1}{2}=1024

y 2 + y + 1 4 = 1024 y^2+ y+ \dfrac{1}{4}=1024

( y + 1 2 ) 2 = 1024 (y+ \dfrac{1}{2})^2=1024

y + 1 2 = 32 y+ \dfrac{1}{2}=32

y = 31.5 y=31.5

Now from the variable change:

x = y 2 1 4 x=y^2- \dfrac{1}{4}

x = ( 31.5 ) 2 1 4 x=(31.5)^2- \dfrac{1}{4}

x = 992 x=992

Aman Deep Singh
Nov 20, 2015

x + x + 1 / 2 + x + 1 / 4 = 2 10 x+\sqrt{x+1/2+\sqrt{x+1/4}} = 2^{10}

Let x + 1 / 4 = t 2 x+1/4=t^{2}

x + 1 / 2 = t 2 + 1 / 4 x+1/2=t^{2}+1/4

t 2 1 / 4 + ( t + 1 / 2 ) 2 = 3 2 2 t^{2}-1/4+\sqrt{(t+1/2)^2}=32^{2}

( t + 1 / 2 ) 2 = 3 2 2 (t+1/2)^{2}=32^2

t = 63 / 2 t=63/2

x + 1 / 4 = 3969 / 4 x+1/4=3969/4

x = 992 \boxed{x=992}

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