Question 1106241
x, length of side of original square piece
5, edge length of each square piece removed, also same as box height
{{{highlight_green((x-2*5)(x-2*5)*5=40)}}}

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{{{5(x-10)(x-10)=40}}}
{{{(x-10)(x-10)=8}}}
{{{x-10=0+- 2*sqrt(2)}}}
{{{x=10+- 2*sqrt(2)}}}
{{{highlight(x=10+2*sqrt(2))}}}





MISREAD PROBLEM DESCRIPTION AND QUESTION***************************************************************
x, edge of each cut out square piece;
also the height of the open box
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{{{cross((5-2x)(5-2x)x=40)}}}

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You might continue with the algebra (rational roots theorem,...); or you could look at some factorizations for 40 instead.
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{{{25x-20x^2+4x^3-40=0}}}
{{{4x^3-20x^2+25x-40=0}}}
In case you prefer, you could try using a graphing tool.





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