Ok guys,

state the surface powers required to produce the following lenses with +5.75D front curves.

a) -0.50DS
b) +0.50DS
c) +1.75DS
d) -1.25DS

i'm not sure if it's as straight forward as calculating the difference, eg. a) +5.75 - -0.50 = +6.25.

Any help is seriously appreciated!