



 
Amber, I am assuming it is to be a circular arch, since you haven't indicated otherwise. 20'11" is 20.917 ' (I want to use decimals). Take a look at http://mathcentral.uregina.ca/QQ/database/QQ.09.07/s/bruce1.html h = r  r cos theta. 20.917 = 25  25 cos theta 20.917 / 25 = 1  cos theta Now the angle you are interested in is actually 2 times theta, so that is 161.2 degrees. The circumference of a circle is twice the radius times Pi. So in your case the full circle would be 2 x 25 x 3.14159 feet = 157.0795 feet. Since you want 161.2 / 360 degrees of a full circle, your arc is ( 161.2 / 360 ) x 157.0795 feet = 70.337 feet, which is about 70'4". Cheers,  


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