Question 468155
First I will give solution in general terms then plug in known heights of 3.5" and 8".
Let b,h,d be base,diagonal, and height of the ramp
Let A be the angle, where A < 10
Using trig relationships:
{{{sin A = h/d}}}
Multiply by d on both sides
{{{d*sin A = h}}}
Divide by sin A on both sides
{{{d = h/sin A}}}
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{{{tan A = h/b}}}
Multiply by b on both sides
{{{b*tan A = h}}}
Divide by tan A on both sides
{{{b = h/tan A}}}
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Now you can substitute a given height and the desired angle.
Use a scientific calculator to obtain trig functions, ex: tan(10) = .1763
For h = 3.5", A = 10 degrees
{{{b = 3.5/tan 10 = 19.85}}}
{{{d = 3.5/sin 10 = 20.15}}}
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For h = 8", A = 10 degrees
{{{b = 8/tan 10 = 45.37}}}
{{{d = 8/sin 10 = 46.07}}}
Note if you need to decrease the angle, the lengths of b,d will increase 

Hope that helps, 
good luck with the ramps