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Geometry Difficulty 3.6 AMC 10/12 Find the answer China

If real numbers xx and yy satisfy (x+5)2+(y12)2=142(x+5)^2 + (y-12)^2 = 14^2, then the minimum value of x2+y2x^2 + y^2 is:

Pick one

Solution

Let x+5=14cosθx+5 = 14\cos\theta and y12=14sinθy-12 = 14\sin\theta, for θ[0,2π)\theta \in [0, 2\pi).
Hence
x2+y2=(14cosθ5)2+(14sinθ+12)2=142+52+122140cosθ+336sinθ=365+28(12sinθ5cosθ) \begin{align*} x^2 + y^2 &= (14\cos\theta - 5)^2 + (14\sin\theta + 12)^2 \\ &= 14^2 + 5^2 + 12^2 - 140\cos\theta + 336\sin\theta \\ &= 365 + 28(12\sin\theta - 5\cos\theta) \end{align*}
=365+28×13sin(θφ)=365+364sin(θφ), \begin{aligned} &=365 + 28 \times 13\sin(\theta - \varphi) \\ &=365 + 364\sin(\theta - \varphi), \end{aligned}
where tanφ=512\tan \varphi = \frac{5}{12}.
So x2+y2x^2+y^2 has the minimum value 11, when θ=3π2+arctan512\theta=\frac{3\pi}{2}+\arctan\frac{5}{12}, i.e. x=513x = \frac{5}{13} and y=1213y = -\frac{12}{13}.
Answer: B.

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