Given: A hexagon ABCDEF
To Prove:
∠ A + ∠B + ∠C + ∠D + ∠E + ∠F = 720°
Construction: Join AD, BE and FC so as to intersect at O.
Proof: In ∆OAB,
∠1 + ∠7 + ∠9 = 180° ...(1)
| Angle sum property of a triangle
In ∆AOBC,
∠2 + ∠10 + ∠11 = 180° ...(2)
| Angle sum property of a triangle
In ∆OCD,
∠3 + ∠12 + ∠13 = 180° ...(3)
| Angle sum property of a triangle
In ∆CDE,
∠4 + ∠14 + ∠15 = 180° ...(4)
| Angle sum property of a triangle
In ∆OEF,
∠5 + ∠16 + ∠17 = 180° ...(5)
| Angle sum property of a triangle
In ∆OFA,
∠6 + ∠18 + ∠8 = 180° ...(6)
| Angle sum property of a triangle
Adding (1), (2), (3), (4), (5) and (6), we get
(∠1 + ∠2 + ∠3 + ∠4 + ∠5 + ∠6)
+ (∠7 + ∠8) + (∠9 + ∠10)
+ (∠11 + ∠12) + (∠13 + ∠14)
+ (∠15 + ∠16) + (∠17 + ∠18)
=1080°
⇒ 360° + ∠A + ∠B + ∠C + ∠D + ∠E + ∠F = 1080°
| ∵ Sum of all the angles round a point is equal to 360°
⇒ ∠A + ∠B + ∠C + ∠D + ∠E + ∠F = 1080° - 360° = 720°