1. The problem statement, all variables and given/known data Find the order of the differential equation of y=C1sin2x+C2cos2x+C3. 2. Relevant equations - 3. The attempt at a solution I read in my book that the order of the differential equation is equal to the number of arbitrary constants but the answer given is 2. Btw I have uploaded two pics with two methods I tried. 1st Pic - http://imgur.com/Py7DTgp 2nd Pic - http://imgur.com/5pdcWq1 In first picture, I calculated upto 3rd differential and obtained a differential equation. In second picture, I differentiated both sides w.r.t. x and then sent the sin2x term, which I was getting in RHS, to LHS and wrote (1/sin2x) as cosec2x. Then I differentiated both sides again w.r.t. x. In this way, both C1 and C2 which remained after calculating 1st derivative become zero. Which one is correct method? If it's neither, then whats the right method?