2013 AMC 12B 第 20 题

All four points lie on the parabola $y = t^2;$ the slope of the chord joining parameters $t_1$ and $t_2$ is $t_1 + t_2$

On this range the parallel sides must be $QR$ and $PS,$ so $\cot x + \sin x = \tan x + \cos x$

Each point $(t, t^2)$ lies on $y = t^2,$ and the chord through parameters $t_1, t_2$ has slope $t_1 + t_2.$ For $135^\circ \lt x \lt 180^\circ,$ both $\cos x$ and $\tan x$ lie between $\cot x$ and $\sin x,$ so $P$ and $S$ sit between $Q$ and $R$ and the parallel sides are $QR$ and $PS.$ Equal slopes give $\cot x + \sin x = \tan x + \cos x.$ Multiplying by $\sin x\cos x$ and simplifying yields $\cos x + \sin x - \sin x\cos x = 0.$ Squaring and using $2\sin x\cos x = \sin 2x$ gives $1 + \sin 2x = \tfrac14\sin^2 2x,$ whose only root in $(-1, 1)$ is $\sin 2x = 2 - 2\sqrt2.$ Thus, the correct answer is A.