In Fig. 6.27,

(i) ∠TPQ = ∠PQR + ∠PRQ
(ii) ∠UQR = ∠QRP + ∠PQR
(iii) ∠PRS = ∠QRP + ∠QPR
Exterior angle property: an exterior angle of a triangle equals the sum of its two opposite interior angles. Applying this to the given figure yields the results above.