If two spheres of radii r1 and r2 cut orthogonally, then the radius of the common circle is
In Δ OPC, cosθ=rr1 In Δ O'PC, sinθ=rr2 As, cos2θ+sin2θ=1 ∴(rr1)2+(rr2)2=1⇒r=r1r2√r21+r22.