How many arbitrary constants will be there in the solution of second order differential equation

If a chemical substance disintegrates at a rate inversely proportional to the square of the amount present at any instant, what is the differential equation governing this law

The normal equation for fitting a straight line y = ax+b is

Which one of the following is a method of curve fitting 

What type of graph sheet is used to get a straight line by plotting (x,y) for the equation y=axⁿ

The sum of the infinite series 3+1+1/3+(1/3)²+.........(1/3)ⁿ+.......... is