I also don't know where it comes from...sign is the factor that makes difference. If a<0, must be: b>0 and c<0 [that ensures f is always decreasing] If a>0, must be: b<0 and c>0 [that ensures f is always increasing] It doesn't matter whether d is positive or negative or zero. --------------- since your assgnmt is due tuesday... I will try to find out more till then.