SAT Math · Systems of equations in two variables · medium

How many real solutions does the system y = -x² + 4x + 1 and y = x - 2 have?

How many real solutions does the system y=−x2+4x+1y = -x^2 + 4x + 1 and y=x−2y = x - 2 have?

Answer and explanation

Answer: B: 2

Set the two expressions for yy equal: −x2+4x+1=x−2-x^2 + 4x + 1 = x - 2. Move everything to one side: 0=x2−3x−30 = x^2 - 3x - 3. The discriminant is b2−4ac=(−3)2−4(1)(−3)=9+12=21b^2 - 4ac = (-3)^2 - 4(1)(-3) = 9 + 12 = 21. Since it is positive, the quadratic has two distinct real roots, so the parabola and the line meet at 2 points, choice B. Choice A (0) would need a negative discriminant. Choice C (1) would need a discriminant of zero, meaning tangency. Choice D (4) is impossible, since a line and a parabola intersect at most twice.

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