Geometric Proof

1
Geometric Proof

• GCSE Higher

2
Prove that the sum of the angles in a triangle is
180
We can use previous knowledge of the properties
of parallel lines
C
EXTEND the line AB to a point G
E
DRAW line BE parallel to AC
A
B
G
3
Prove that the sum of the angles in a triangle is
180
ltCBA ltCBE ltEBG 180º
Angles on a straight line
ltACB ltCBE
Alternate Angles
ltCAB ltEBG
Corresponding Angles
SO ltCBA ltACB ltCAB 180º
The Angle Sum of a triangle is 180º
4
Using this

• could you prove that the exterior angle of a
  triangle is equal to the sum of the two opposite
  interior angles?