The problem defines a function $g(\theta)$ in terms of a determinant $f(\theta)$. We are given $g(\theta) = \sqrt{f(\theta)-1} + \sqrt{f(\pi/2 - \theta) - 1}$, where $f(\theta)$ is a determinant of a $3 \times 3$ matrix. We are also given that $p(x)$ is a quadratic polynomial whose roots are the maximum and minimum values of $g(\theta)$, and $p(2) = 2 - \sqrt{2}$. We need to determine which of the given inequalities involving $p(x)$ are true.

AlgebraDeterminantsTrigonometryQuadratic EquationsOptimizationInequalities
2025/6/11

1. Problem Description

The problem defines a function g(θ)g(\theta) in terms of a determinant f(θ)f(\theta). We are given g(θ)=f(θ)1+f(π/2θ)1g(\theta) = \sqrt{f(\theta)-1} + \sqrt{f(\pi/2 - \theta) - 1}, where f(θ)f(\theta) is a determinant of a 3×33 \times 3 matrix. We are also given that p(x)p(x) is a quadratic polynomial whose roots are the maximum and minimum values of g(θ)g(\theta), and p(2)=22p(2) = 2 - \sqrt{2}. We need to determine which of the given inequalities involving p(x)p(x) are true.

2. Solution Steps

First, let's compute f(θ)f(\theta):
f(θ)=1sinθ1sinθ1sinθ1sinθ1f(\theta) = \begin{vmatrix} 1 & \sin \theta & 1 \\ -\sin \theta & 1 & \sin \theta \\ -1 & -\sin \theta & 1 \end{vmatrix}
f(θ)=1(1+sin2θ)sinθ(sinθ+sinθ)+1(sin2θ+1)f(\theta) = 1(1+\sin^2 \theta) - \sin \theta (-\sin \theta + \sin \theta) + 1(\sin^2 \theta + 1)
f(θ)=1+sin2θ+0+sin2θ+1=2+2sin2θf(\theta) = 1 + \sin^2 \theta + 0 + \sin^2 \theta + 1 = 2 + 2 \sin^2 \theta
Now, let's compute f(π/2θ)f(\pi/2 - \theta):
f(π/2θ)=2+2sin2(π/2θ)=2+2cos2θf(\pi/2 - \theta) = 2 + 2 \sin^2(\pi/2 - \theta) = 2 + 2 \cos^2 \theta
Therefore, g(θ)=2+2sin2θ1+2+2cos2θ1g(\theta) = \sqrt{2 + 2 \sin^2 \theta - 1} + \sqrt{2 + 2 \cos^2 \theta - 1}
g(θ)=1+2sin2θ+1+2cos2θg(\theta) = \sqrt{1 + 2 \sin^2 \theta} + \sqrt{1 + 2 \cos^2 \theta}
We need to find the maximum and minimum values of g(θ)g(\theta).
Note that g(π/2θ)=1+2cos2θ+1+2sin2θ=g(θ)g(\pi/2 - \theta) = \sqrt{1 + 2 \cos^2 \theta} + \sqrt{1 + 2 \sin^2 \theta} = g(\theta). We can also calculate the derivative:
g(θ)=4sinθcosθ21+2sin2θ4sinθcosθ21+2cos2θ=2sinθcosθ(11+2sin2θ11+2cos2θ)g'(\theta) = \frac{4 \sin \theta \cos \theta}{2 \sqrt{1 + 2 \sin^2 \theta}} - \frac{4 \sin \theta \cos \theta}{2 \sqrt{1 + 2 \cos^2 \theta}} = 2 \sin \theta \cos \theta (\frac{1}{\sqrt{1+2\sin^2\theta}} - \frac{1}{\sqrt{1+2\cos^2\theta}})
Setting g(θ)=0g'(\theta) = 0, either sinθcosθ=0\sin \theta \cos \theta = 0, or 1+2sin2θ=1+2cos2θ\sqrt{1+2\sin^2\theta} = \sqrt{1+2\cos^2\theta}
If sinθcosθ=0\sin \theta \cos \theta = 0, then sinθ=0\sin \theta = 0 or cosθ=0\cos \theta = 0, so θ=0\theta = 0 or θ=π/2\theta = \pi/2.
If 1+2sin2θ=1+2cos2θ\sqrt{1+2\sin^2\theta} = \sqrt{1+2\cos^2\theta}, then sin2θ=cos2θ\sin^2 \theta = \cos^2 \theta, so tan2θ=1\tan^2 \theta = 1, meaning tanθ=±1\tan \theta = \pm 1.
Since θ[0,π/2]\theta \in [0, \pi/2], θ=π/4\theta = \pi/4.
When θ=0\theta = 0, g(0)=1+1+2=1+3g(0) = \sqrt{1} + \sqrt{1+2} = 1 + \sqrt{3}.
When θ=π/2\theta = \pi/2, g(π/2)=1+2+1=3+1g(\pi/2) = \sqrt{1+2} + \sqrt{1} = \sqrt{3} + 1.
When θ=π/4\theta = \pi/4, g(π/4)=1+2(1/2)+1+2(1/2)=2+2=22g(\pi/4) = \sqrt{1 + 2(1/2)} + \sqrt{1 + 2(1/2)} = \sqrt{2} + \sqrt{2} = 2 \sqrt{2}.
Since 1+31+1.732=2.7321+\sqrt{3} \approx 1 + 1.732 = 2.732 and 222(1.414)=2.8282\sqrt{2} \approx 2(1.414) = 2.828, the maximum value is 222\sqrt{2} when θ=π/4\theta = \pi/4, and the minimum value is 1+31+\sqrt{3} when θ=0\theta=0 or θ=π/2\theta = \pi/2. This is incorrect because 222\sqrt{2} is the min and 1+31 + \sqrt{3} is the max.
When θ=0\theta = 0, g(0)=1+3g(0) = 1 + \sqrt{3}. When θ=π4\theta = \frac{\pi}{4}, g(π4)=2+2=22g(\frac{\pi}{4}) = \sqrt{2} + \sqrt{2} = 2\sqrt{2}. When θ=π2\theta = \frac{\pi}{2}, g(π2)=3+1g(\frac{\pi}{2}) = \sqrt{3} + 1.
Thus, minimum = 222\sqrt{2} and maximum is 1+31 + \sqrt{3}.
Since the roots of p(x)p(x) are 222\sqrt{2} and 1+31+\sqrt{3}, p(x)=a(x22)(x(1+3))p(x) = a(x-2\sqrt{2})(x-(1+\sqrt{3})) for some constant aa.
We are given p(2)=22p(2) = 2 - \sqrt{2}. Thus, a(222)(2(1+3))=22a(2-2\sqrt{2})(2 - (1+\sqrt{3})) = 2 - \sqrt{2}
a(222)(13)=22a(2-2\sqrt{2})(1-\sqrt{3}) = 2-\sqrt{2}
a(2(12))(13)=22a(2(1-\sqrt{2}))(1-\sqrt{3}) = 2-\sqrt{2}
a(12)(13)=12/2a(1-\sqrt{2})(1-\sqrt{3}) = 1 - \sqrt{2}/2
2a(12)(13)=222a(1-\sqrt{2})(1-\sqrt{3}) = 2-\sqrt{2}
Since a(12)(13)=222=122a(1-\sqrt{2})(1-\sqrt{3}) = \frac{2-\sqrt{2}}{2} = 1 - \frac{\sqrt{2}}{2}, a=122(12)(13)=222(12)(13)a = \frac{1-\frac{\sqrt{2}}{2}}{(1-\sqrt{2})(1-\sqrt{3})} = \frac{\frac{2-\sqrt{2}}{2}}{(1-\sqrt{2})(1-\sqrt{3})}.
It turns out the roots are minimum 222\sqrt{2} and maximum 1+31+\sqrt{3} so the shape of the graph is upside down and a is negative. Thus p(x)=a(x(1+3))(x22)p(x)=a(x-(1+\sqrt{3}))(x-2\sqrt{2}).
p(2)=22p(2)=2-\sqrt{2} so a(213)(222)=22a(2-1-\sqrt{3})(2-2\sqrt{2})=2-\sqrt{2}
a(13)(222)=22a(1-\sqrt{3})(2-2\sqrt{2})=2-\sqrt{2}
a(13)2(12)=22a(1-\sqrt{3})2(1-\sqrt{2})=2-\sqrt{2}
a=222(13)(12)=2(21)2(13)(12)=22(13)=2(3+1)2(2)=2(3+1)4a=\frac{2-\sqrt{2}}{2(1-\sqrt{3})(1-\sqrt{2})}=\frac{\sqrt{2}(\sqrt{2}-1)}{2(1-\sqrt{3})(1-\sqrt{2})}=-\frac{\sqrt{2}}{2(1-\sqrt{3})}=\frac{\sqrt{2}(\sqrt{3}+1)}{2(-2)}=-\frac{\sqrt{2}(\sqrt{3}+1)}{4}
Since a<0a < 0, p(x)p(x) is a downward-facing parabola. The roots are 222.8282\sqrt{2} \approx 2.828 and 1+32.7321+\sqrt{3} \approx 2.732.
p(x)=a(x22)(x13)p(x) = a(x-2\sqrt{2})(x-1-\sqrt{3}).
A. p(3+24)<0p(\frac{3+\sqrt{2}}{4}) < 0. 3+243+1.41444.41441.1\frac{3+\sqrt{2}}{4} \approx \frac{3+1.414}{4} \approx \frac{4.414}{4} \approx 1.1. Since 3+24\frac{3+\sqrt{2}}{4} is less than the roots, we expect p(x)p(x) is negative.
B. p(1+324)>0p(\frac{1+3\sqrt{2}}{4}) > 0. 1+3241+3(1.414)41+4.24245.24241.3\frac{1+3\sqrt{2}}{4} \approx \frac{1+3(1.414)}{4} \approx \frac{1+4.242}{4} \approx \frac{5.242}{4} \approx 1.3.
C. p(5214)>0p(\frac{5\sqrt{2}-1}{4}) > 0. 52145(1.414)147.07146.0741.5\frac{5\sqrt{2}-1}{4} \approx \frac{5(1.414)-1}{4} \approx \frac{7.07-1}{4} \approx \frac{6.07}{4} \approx 1.5.
D. p(524)<0p(\frac{5-\sqrt{2}}{4}) < 0. 52451.41443.58640.8965\frac{5-\sqrt{2}}{4} \approx \frac{5-1.414}{4} \approx \frac{3.586}{4} \approx 0.8965.
Since p(22)=0p(2\sqrt{2})=0 and p(1+3)=0p(1+\sqrt{3})=0, and knowing a is negative (a<0a<0) then values outside of this range make p(x)<0p(x)<0. Values inside this range make p(x)>0p(x)>0.
p(3+24)<0p(\frac{3+\sqrt{2}}{4})<0 since 3+241.1\frac{3+\sqrt{2}}{4} \approx 1.1 which is less than 2.7322.732 and 2.8282.828.
p(524)<0p(\frac{5-\sqrt{2}}{4})<0 since 5240.9\frac{5-\sqrt{2}}{4} \approx 0.9 which is less than 2.7322.732 and 2.8282.828.
Options A and D are correct.

3. Final Answer

A, D

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