Question

for combination of thin lens in contact , for first lens 1/v-1/u=1/f which is lens formula but as u is negtive then why don't we use postive sign i.e. 1/v+1/u=1/f?

Answer 1

I agree with you but we are deriving a general form for combination of thin lens in contact and not substituting any numeric value in that so we had used only standard general form to make it eligible/applicable to all cases.