Calculate the angles of the quadrilateral in the picture and also the angles between their diagonals:
In Δ FBD
∠ BFD + ∠ FBD + ∠ BDF = 180° (angle sum property)
⇒ ∠ BFD + 50 + 30 = 180°
⇒ ∠ BFD = 180° – 80° = 100°
Also, ∠ CFE = ∠ BFD =100° (∵ ∠ BFD and ∠ CFE are vertically
opp. Angle)
Since, EFD is a line
∠ BFD + ∠ BFE = 180° (sum of angle on a straight line is 180°)
⇒ 100 + ∠ BFE = 180°
⇒ ∠ BFE = 180° – 100° = 80°
Also, ∠ CFD = ∠ BFE = 80° (∵ ∠ BFE and ∠ CFD are vertically opp. Angle)
In Δ FBE
∠ BFE + ∠ FBE + ∠ BEF = 180° (angle sum property)
⇒ ∠ FBE + 80 + 45 = 180°
⇒ ∠FBE = 180° – 125° = 55°
Thus, ∠ DBE = ∠ FBD + ∠FBE
⇒ ∠ DBE = 30 + 55 = 85°
In quad CDBE
∠ DCE + ∠ DBE = 180° (∵ if all four vertices of a quadrilateral are
on circle then opposite angle are supplementary)
⇒ ∠ DCE + 85 = 180°
⇒ ∠ DCE = 180° – 85° = 95°
Also,
∠ CBD = ∠ DEC = 30° (∵ angle in a same segment are equal)
∠ CBE = ∠ CDE = 55° (∵ angle in a same segment are equal)
Thus, ∠ CDB = ∠ CDE + ∠ BDE
⇒ ∠ CDB = 55 + 50 = 105°
Thus, ∠ CEB = ∠ CED + ∠ BED
⇒ ∠ CEB = 30 + 45 = 75°