<span>Tha answer is 3) if a is positive, the parabola opens upward. You can see that when you imagine what happens when x is large. In that case bx + c is small compared to ax^2, and the value of the function is dominated by the terms ax^2. x^2 is positive always, so the sign of ax^2 is given by a. Then if a is positive the extreme values of the function (x positively large and x negatively large) are positive, which is the case when the parabola opens upward.</span>
