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Algebra Difficulty 3.2 AMC 10/12 Find the answer

Which of the following inequalities has a solution set of RR?

A: x2+6x+10>0x^{2}+6x+10 \gt 0

B: x2+25x+5>0{x}^{2}+2\sqrt{5}x+5 \gt 0

C: x2+x2<0-x^{2}+x-2 \lt 0

D: 2x23x3<02x^{2}-3x-3 \lt 0

Multiple choice: answer with the letter of the option you want.

Solution

To analyze the solution sets of the given inequalities, we need to check the discriminant Δ\Delta and the coefficient of x2x^2 to determine the nature of the roots and consequently, the solution set of each inequality.

### Option A: x2+6x+10>0x^{2}+6x+10 \gt 0

1. Calculate the discriminant: Δ=b24ac=624×1×10=3640=4<0\Delta = b^{2}-4ac = 6^{2}-4\times 1\times 10 = 36 - 40 = -4 \lt 0.
2. Since Δ<0\Delta \lt 0 and the coefficient of x2x^2 (which is 11) is positive, the parabola opens upwards and does not intersect the x-axis. Therefore, the inequality holds for all real numbers.

A is correct\boxed{A \text{ is correct}}.

### Option B: x2+25x+5>0{x}^{2}+2\sqrt{5}x+5 \gt 0

1. Calculate the discriminant: Δ=(25)24×1×5=2020=0\Delta = (2\sqrt{5})^{2}-4\times 1\times 5 = 20 - 20 = 0.
2. Since Δ=0\Delta = 0, the equation has a double root: x1=x2=5{x}_{1}={x}_{2}=-\sqrt{5}.
3. The parabola opens upwards (coefficient of x2x^2 is positive), touching the x-axis at one point. Hence, it is positive for all xx except at x=5x=-\sqrt{5}.

B is incorrect\boxed{B \text{ is incorrect}}.

### Option C: x2+x2<0-x^{2}+x-2 \lt 0

1. Calculate the discriminant: Δ=124×(1)×(2)=18=7<0\Delta = 1^{2}-4\times (-1)\times (-2) = 1 - 8 = -7 \lt 0.
2. Since Δ<0\Delta \lt 0 and the coefficient of x2x^2 (which is 1-1) is negative, the parabola opens downwards and does not intersect the x-axis. Therefore, the inequality holds for all real numbers.

C is correct\boxed{C \text{ is correct}}.

### Option D: 2x23x3<02x^{2}-3x-3 \lt 0

1. Calculate the discriminant: Δ=(3)24×2×(3)=9+24=33>0\Delta = (-3)^{2}-4\times 2\times (-3) = 9 + 24 = 33 \gt 0.
2. Since Δ>0\Delta \gt 0, the equation has two distinct real roots: x=3±334x=\frac{3 \pm \sqrt{33}}{4}.
3. As the parabola opens upwards (coefficient of x2x^2 is positive), the inequality holds between the roots.

D is incorrect\boxed{D \text{ is incorrect}}.

Therefore, considering the analysis above, the correct options are AC\boxed{AC}.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.