If we have a function y = r then we can differentiate it to obtain dyldx = 2x. We can define integration as the mathematical process which reverses differentiation, i.e. finds the function in this case which on being differentiated gives 2x. Thus integrating 2x should give us r. However, the derivative ofr + 3 is also 2x. Thus in the integration of 2x we are not sure whether there is a constant term or not. Hence a constant C has to be added to the result. Thus the outcome of the integration of r is 2x + C. The integral in such circumstances is, because of the constant, referred to as the indefinite integral.