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Algebra Difficulty 4.2 AIME Find the answer United States

Let xx be the least real number greater than 11 such that sinx=sin(x2)\sin x = \sin(x^2), where the arguments are in degrees. What is xx rounded up to the closest integer?

Pick one

Solution

Answer (B): The given condition means that either xx and x2x^2 represent the same angle or they represent supplementary angles. In the first case x2=x+360kx^2 = x + 360k for some integer kk. By the quadratic formula,
x=1±1+1440k2. x = \frac{1 \pm \sqrt{1 + 1440k}}{2}.
The least value of x>1x > 1 occurs when k=1k = 1 and the plus sign is used, in which case
x=1+14412. x = \frac{1 + \sqrt{1441}}{2}.
In the second case x2+x=180(2k+1)x^2 + x = 180(2k + 1) for some integer kk. By the quadratic formula,
x=1±1720(2k+1)2. x = \frac{-1 \pm \sqrt{1 - 720(2k + 1)}}{2}.
The least value of x>1x > 1 occurs when k=1k = -1 and the plus sign is used, in which case
x=1+7212. x = \frac{-1 + \sqrt{721}}{2}.
Because 272=729>72127^2 = 729 > 721 and 252=625<72125^2 = 625 < 721, the value of xx rounds up to 1+272=13\frac{-1+27}{2} = 13, which is smaller than the value of xx obtained from the first case.

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