Topic: point e and point d are on the extension lines of BA and CA beside △ABC, and CF and EF are equally divided into ∠ACB and ∠AED respectively. If ∠ B = 76 and D = 42, find the size of ∠ F 。
Solution: let CF and AB intersect at point g,
∠EAD = 180-∠E-∠D = 180-∠E-42 = 138-∠E
∠CAB = 180-∠C-∠B = 180-∠C-76 = 104-∠C
∠∠EAD =∠CAB (equal to the vertex angle)
∴ 138-∠E= 104-∠C = ∠ E-∠ C = 138- 104 = 34.
At delta △EFG,
∠∠EGF =∠∠CGB (equal to vertex angle)
= 180-∠B -( 1/2)∠C
= 180-76-( 1/2)∠C
= 104-( 1/2)∠C,
∴∠f= 180-( 1/2)∠e-∠egf
= 180-( 1/2)≈E-( 104-( 1/2)≈C)
= 76-( 1/2)≈E+( 1/2)≈C)
=76-( 1/2)*(∠E-∠C)
=76-( 1/2)*34
=59