What is the radius of a circle that changes 1.5" in 64" (straight)? thanks, ron

Question

I am trying to cut an arch on the bottom chord of a truss

Answer 1

Draw your circle edge and a tangent, a radius (R1) to the tangent point (T), and another radius (R2) to a point 64" along the tangent line (Point A). Then add a chord through T and the intersection of R2 and the circle (Point B). Bisect the angle formed by R1 and R2, with the half-angle called θ. Then cos θ = (r-0.75)/r and sin θ = 32/r. Cos²θ + sin²θ=1, solve to get r~=4096/1.5, you do the math.