Given: AD = CE To prove: ∠ABC = 1/2(∠DOE – ∠AOC) In △AOD and △COE AD = CE (Given) AO = OC and DO = OE (Radii of same circle) ∴ △AOD ≅ △COE (By SSS congruence criterion) ⇒ ∠1 = ∠3, ∠2 = ∠4 (CPCT) ....(i) But OA = OD and OC = OE ⇒ ∠1 = ∠2 and ∠3 = ∠4 .....(ii) From (i) and (ii), we have ∠1 = ∠2 = ∠3 = ∠4 (= x say) Also, OA = OC and OD = DE ⇒ ∠7 = ∠8 (= z say) and ∠5 = ∠6 (= y say) Now, ADEC is a cyclic quadrilateral ⇒ ∠DAC + ∠DEC = 180° ⇒ x + z + x + y = 180° ⇒ y = 180° - 2x - z ...(iii) In △DOE,∠DOE = 180° - 2y and in △AOC, ∠AOC = 180° - 2z ∴ ∠DOE - ∠AOC = (180° - 2y) - (180° - 2z) = 2z - 2y = 2z - 2(180° - 2x - z) (Using (iii)) = 4z + 4x - 360° ....(iv) Again, ∠BAC + ∠CAD = 180° ⇒ ∠BAC = 180°– (z + x) ....(v) Similarly, ∠BCA = 180°– (z + x) ....(vi) In △ABC, ∠ABC = 180° - ∠BAC - ∠BCA = 180° - 2[180° - (z - x)] (Using (v) and (vi)) = 2z + 2x - 180° = 1/2(4z + 4x - 360°) ....(vii) From (iv) and (vii), we have ∠BAC = 1/2(∠DOE - ∠AOC)