# Work done in inserting capacitor

We apply force to insert a dielectric slowly between capacitor .

While inserting , we are assuming charge is constant.

Now my sir told that

Work done by external agent $= Q^2 /2C[(1/k)-1].$

I could not why it is negative as according to me this should be the work done by force to insert it. And the direction slab displaces and force is same , so it should be positive m