It depends also on the rate at which currents rise and fall, and this is indicated by the differential coefficient dC / dt.
Thus he differentiated with respect to x and equated the differential coefficient to zero.
When this condition is satisfied y is a function of x which has a differential coefficient.
The only way of 544 finding out whether this condition is satisfied or not is to attempt to form the differential coefficient.
It appears that we can always find the area A if we know a function F(x) which has ƒ(x) as its differential coefficient.