Question

the side of bc of ∆ abc is produced ob both sides prove that the sum of the exterior angles so formed is greater than angle a by two right angle

Answer 1

MULTIPLYING 1 WITH 2,WEGET,
2<A+2<B+2<C=360*_______5
EQUATING 4 AND 5 WE GET,
<B+<C+ext<B+ext<C=360*_______4
ADDING 2 AND 3,
<C+ext<C=180* (  "  )________3
<B+ext<B=180* (LINEAR PAIR)______2
<A+<B+<C=180*_______1
in tri< abc,
         To Prove:               ext.angle B {b}  +ext.angle C {c}  = angle A + 180 degree
A                                   In triangle ABC,
                                                     <A+<B+<C=180....(1)       [by angle sum property]