Find the number of one-one function f(A) - A satisfying f(f(i)) =i. Given that A has four elements (1,2,3,4 ) .......... Sir plz explain briefly from basics

the diameter of a roller is 84 cm and its length is 120 cm. it takes 500 complete revolution to  move once over to level a playground. find the area of the playground in square m?

Find dy/DX when y=x^sinx+sin-1√x.

 

Please give me the solution of this problem. 

Solve the differential equation y’’+2ty’-4y=1, y(0)=y’(0)=0.