Question

basic proportionality theorem

Answer 1

see in 10 text book of similar triangles theorem 9.1

Answer 2

it is the theorem that states that if a linbe is drawn parallel to one side then it divides the other two sides in the same ratio

for eg id de is parallel to bc then ae/ec=ad/dc

or ab/ad=ac/ae

or ab/db=ac/ec

or their vice versa