Then ∠ABD is the minimum angle α.

In Rt△ABD, sin α = ad/ab = 30/90 = 1/3.

(2) In the same way, the tangent from do⊙E to c.

At this point, sin ∠ ECA = 30/75 = 6/ 15.

According to the calculator, Arcsin = 6/ 15 ≈ 23.58.

Well, this problem is ok to do, but it is a bit difficult to type.