definite integration?

Question

In my book it is written that:
[-1/r](where upper limit is r and lower limit is ∞]
then write,
[-1/r+1/∞]
But i don't understand how [-1/r+1/∞] comes ?any rule?
please explain me.

Answer 1

-1/r on application of the limits will be 1/r-(1/-infinity)
which is equal to 1/r+1/infinity
when youapply the limits you substitute the value of the limits for the variable