How did senior one mathematics, such as 4-4a=0 in the answer, come from? Online emergency

The triangle in front is a discriminant. Just remember that the discriminant is b? -4ac, where a is a quadratic coefficient, b is a linear coefficient and c is a constant term. Just plug it into the formula. In this question, ax? Is a, 2x is b, 1 is c, and it is substituted into the discriminant formula (x does not need to be substituted, only numbers are brought in) ①2? -4×a× 1 = 0②4-4a = 0③-4a =-4④a = 1

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