Question 318522
{{{drawing(300,300,-10,10,-10,10,line(-3,0,3,0),line(-3,0,0,4.24),line(0,0,0,4.24),locate(-3,3,s),locate(2.5,3,s),locate(0.5,2,h), locate(0,-0.5,s),line(-0.5,0,-0.5,0.5),line(-0.5,0.5,0,0.5),line(3,0,0,4.24))}}}
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{{{A=(1/2)bh}}}
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Find h using the Pythagorean theorem.
{{{h^2=s^2-(s/2)^2}}}
{{{h^2=s^2/2}}}
{{{h=(sqrt(2)/2)*s}}}
{{{A=(1/2)s(sqrt(2)/2)s}}}
{{{A=(sqrt(2)/4)s^2}}}
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Find {{{s}}} using the perimeter.
{{{P=3s=21}}}
{{{s=7}}}
Substitute,
{{{A=(49*sqrt(2))/4}}} or approximately,
{{{A=17.324}}} 
The area is 17.3 sq. inches.