Question 1073740
*[illustration L19.JPG].
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{{{H=18+2X}}}
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The interior angles of the octagon are 135 degrees. 
So the X-X-18 triangle is a 45-45-90 triangle so then,
{{{X^2+X^2=18^2}}}
{{{2X^2=18^2}}}
{{{X=18/sqrt(2)}}}
So,
{{{H=18+36/sqrt(2)}}}
{{{H=18+18sqrt(2)}}}
{{{H=18(1+sqrt(2))}}}{{{in}}}