I substituted every answer for the first question of multiple choice questions. I can't understand the standard answer. Please tell me why you wrote the standard answer.

According to the options, this is obviously a quadratic function. The root of the equation is 2 -4, and the parabolic opening is downward. A<0, according to the relationship between root and coefficient. (unary quadratic equation ax 2+bx+c = 0.

When the discriminant △=b? When -4ac is not equal to 0, the equation has two unequal real roots. When the equation has roots, let two roots be X 1, x2, X 1+x2 =-b/a, X 1 * x2 = c/a, and the sum of the two roots is equal to the reciprocal of the ratio of the coefficient of the first term to the coefficient of the second term. C