唔識做,,,計角度

設∠CTB = x
∠CBT = x (等腰Δ底角)
∴∠OCT = x + x = 2x (Δ外角)

∠OTB = 90(半徑⊥切線)
∠OTC = 90 - x

OT = OC (半徑)
∠OCT = ∠OTC (等腰Δ底角)
2x = 90 - x
x = 30
∴∠OTC = ∠OCT = 60
∴∠TOC = 60 (Δ內角和)

11.
Let ∠ODT = x
OD = OT(半徑)
∴∠OTD = x(等腰Δ底角)
∠OTB = 90 (半徑⊥切線)
∠CBT = 180 - x - (x + 90) (Δ內角和)
∠CBT = 90 - 2x

∠COT = x+x = 2x (Δ外角)
∠OTC = ∠OCT(等腰Δ底角)
∠OCT = (180 - 2x)2
∠OCT = 90 - x

∠CTB + ∠CBT = ∠OCT (Δ外角)
∠CTB + (90 - 2x) = 90 - x
∠CTB = x
∴∠ODT = ∠CTB