>>757
AC=2R
BC=x
CD=y
として

cosα = a/2R　　　(1)

cos(60-α) = b/2R
(1/2)cosα + (√3/2)sinα = b/2R (2)

sin(α) = x/2R　　　(3)

sin(60-α) = y/2R
(√3/2)cosα - (1/2)sinα = y/2R (4)

(2)から
(1/2)*(a/2R) + (√3/2)*(x/2R) = b/2R
x = (1/√3)(2b - a)

(4)から
(√3/2)cosα - (1/2)sinα = y/2R
(√3/2)*(a/2R) - (1/2)(x/2R) = y/2R

以下略