turunan parsial terhadap x dan y dari z = (x+y) len(xy)

z = uv
z' = u'v + uv'

z = (x + y) ㏑ (xy)
∂z / ∂x = (1 + 0) ㏑ (xy) + (x + y) [1 / (xy) y]
           = ㏑ (xy) + 1/x (x + y)
           = ㏑ (xy) + 1 + y/x

z = (x + y) ㏑ (xy)
∂z / ∂y = (0 + 1) ㏑ (xy) + (x + y) [1 / (xy) x]
           = ㏑ (xy) + (1 / y) (x + y)
           = ㏑ (xy) + x/y + 1