add the fractions 4/x and (x+1)/(x-5)
Let's sum the fractions: 4x + x + 1x -5
4·(x -5) + (x + 1)·xx·(x -5)
4·(x -5) = 4x -20(x + 1)·x = x2 + x
(4x -20) + (x2 + x)x·(x -5)
4x -20 + x2 + x = 5x -20 + x2
5x -20 + x2x·(x -5)
Result:   x2 + 5x -20x·(x -5)