Connect OD, then ∠OAD=∠ODA (isosceles triangle)

And δδABC is an isosceles triangle, then ∠BAC=∠BCA.

So ∠ODA=∠BCA,

Because DF⊥BC, ∠BCA+∠CDF=90 degrees

But ∠ODA+∠CDF= 90 degrees, that is ∠ODE=90 degrees, that is, the certificate.

(2) Find the cosine (cosine theorem) of ∠BOD first, then the sine of ∠BOD, and then the reciprocal, which is what you want.