Hi Purushottam,
For 40th, please check Example 29 of Chapter 3 Class 11 here https://www.teachoo.com/2233/586/Example-29---Prove-cos2-x---cos2-(x---pi-3)---cos2-(x---pi-3)/category/Examples/
For 50th(b), please check Example 22(ii) of Chapter 13 Class 11 here https://www.teachoo.com/2868/645/Example-1---Find-limits--lim-x--1--x3---x2---1----Class-11/category/Examples/
Written on Feb. 25, 2017, 8:38 p.m.