Question 13 Marks
If the base of an isosceles triangle is produced on both sides, prove that the exterior angles so formed are equal to each other.
Answer
View full question & answer→ED is a straight line segment and B and C an points on it.$\angle\text{EBC}=\angle\text{BCD}=\text{straight angle}=180^\circ$
$\angle\text{EBA}+\angle\text{ABC}=\angle\text{ACB}+\angle\text{ACD}$
$\angle\text{EBA}+\angle\text{ACD}+\angle\text{ACB}-\angle\text{ABC}$
$\angle\text{EBA}=\angle\text{ACD}$ [From (i) ABC = ACD]
$\angle\text{ABE}=\angle\text{ACD}$
Hence proved.
$\angle\text{EBA}+\angle\text{ABC}=\angle\text{ACB}+\angle\text{ACD}$
$\angle\text{EBA}+\angle\text{ACD}+\angle\text{ACB}-\angle\text{ABC}$
$\angle\text{EBA}=\angle\text{ACD}$ [From (i) ABC = ACD]
$\angle\text{ABE}=\angle\text{ACD}$
Hence proved.


Since, two sides and angle between them are equal, therefore triangle ABC and DEF are congruent.