So you want to pair them up (1-2)+(3-4)+(5-6)..+(99-100) And write the answer for each pair (-1)+(-1)+(-1)..+(-1) When looking at this you can see that each pair equals -1 So you divide 100 by 2 and multiply that by the -1 100/2=50 50*-1=-50 Answer is 50
You would use the same method if you were going up to a number like a 500 The answer would be -250
