Hi Basant,
You can check Theorem 8.1 from here https://www.teachoo.com/4274/1043/Theorem-8.1---Class-9th---Diagonal-of-a-parallelogram-divides-it-into-two-congruent-triangles/category/Theorems/
To view answers of all NCERT questions, visit https://www.teachoo.com/maths/