y=(x+1)(x-2)/[(x-1)(x+2)], ln(y)=ln(x+1)+ln(x-2)-ln(x-1)-ln(x+2).
(1/y)dy/dx=1/(x+1)+1/(x-2)-1/(x-1)-1/(x+2),
(1/y)dy/dx=[(x2-4)(x-1)+(x2-1)(x+2)-(x2-4)(x+1)-(x2-1)(x-2)]/[(x2-1)(x2-4)],
(1/y)dy/dx=[(x2-4)(x-1-x-1)+(x2-1)(x+2-x+2)]/[(x2-1)(x2-4)],
(1/y)dy/dx=(-2x2+8+4x2-4)/(x4-5x2+4),
(1/y)dy/dx=2(x2+2)/(x4-5x2+4),
dy/dx=[2(x2+2)/(x4-5x2+4)][(x+1)(x-2)/[(x-1)(x+2)]],
dy/dx=2(x2+2)/[(x-1)2(x+2)2].