-x+2x -3x+4x -5x+...+100x =
= -x+2x -3x+4x -5x+6x ... -99x +100x =
= x(-1+2 -3+4 -5+6 ... -99 +100) =
= x(1 +1 +1 ... +1) =
= x · 50
= 50x
