Child Labor 13

Calculus Level 4

Solve: x = x + x 3 + x 4 \Huge x = \sqrt x + \sqrt[3]{x} + \sqrt[4]{x}

Note: Answer correct to 5 decimal places. x > 0 x >0


The answer is 5.73058.

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2 solutions

Substitute x = y 12 x=y^{12} to get rid of those pesky radicals.Then the equation becomes: y 12 = y 6 + y 4 + y 3 y 12 y 6 y 4 y 3 = 0 \begin{aligned} y^{12}=y^6+y^4+y^3\\ y^{12}-y^6-y^4-y^3=0\\ \end{aligned} Factoring the equation,we get y 3 ( y + 1 ) ( y 8 y 7 + y 6 y 5 + y 4 y 3 1 ) y^3(y+1)(y^8-y^7+y^6-y^5+y^4-y^3-1) .The triple root y = 0 y=0 is discarded since that would give x = 0 12 = 0 x=0^{12}=0 but we are given that x > 0 x>0 .The root y = 1 y=-1 is also discarded since x = ( 1 ) 12 = 1 x=(-1)^{12}=1 does not satisfy the equation.Solving y 8 y 7 + y 6 y 5 + y 4 y 3 1 y^8-y^7+y^6-y^5+y^4-y^3-1 by any root-finding algorithm gives two real roots: y = 1.1566 y=1.1566 or y = 0.698644 y=-0.698644 . x = ( 0.698644 ) 12 x=(-0.698644)^{12} does not satisfy the equation,hence the answer is x = ( 1.1566 ) 12 5.73057 x=(1.1566)^{12}\approx \boxed{5.73057} to 5 5 decimal places.

I did same!!

Dev Sharma - 5 years, 7 months ago

Graphing and using zoom box, there are two solutions, 0 and 5.7305778. I used ti -83 plus.

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