the formula of newton law of gravitation is GMm/r^2or -GMm/r^2 why got book put in the negative sign while some no?

The negative sign is to account for the direction of the gravity as a vector quantity where the vector field lines are pointing towards the center.

so did i have to put in the negative sign in the derivation of gravitational potential energy

The most correct form for gravitational "force" would have a negative sign because gravity is technically a vector quantity.

Gravitational potential "energy" is a scalar quantity. Therefore the sign does not really matter. However, it doesn't hurt to ask which sign is usually preferred if you are concerned about this for answering exam questions.

so which sign is usually preferred in the examination?

to avoid marked deducted, better with negative sign

1. For the GMm/r^2, with or without negative sign doesn't matter. This formula tells you about the magnitude of the force that exist between 2 point masses only. To account for the direction of the force, sign convention is employed such as force to the right is postive"+". But in your examination, GMm/r^2 without negative sign is preferred because usually the question requires only the magnitude of the force between them.

2. Gravitational potential ALWAYS has a negative "-" sign in the formula. The negative sign is from the derivation.

3. Gravitational potential ENERGY is a scalar quantity. Thus the sign doesn't really matter as said by @maru&box.