Q: Let y = 4 x^2.
Find the change in y, \Delta y when x= 4 and \Delta x = 0.2
Find the differential dy when x= 4 and dx = 0.2
dy/dx = 8x
dy = 8x * dx
dy = (8*4)(.2) = 6.4
But 6.4 is the correct answer only for dy and not change of y, ain't they the same thing?