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Geometry Level 3

If (sin x)+(cos x)=2, find (sin 2x)..


The answer is 3.

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2 solutions

Lu Chee Ket
Jan 28, 2015

(Sin x + Cos x) exceeds Sqrt (2) means x are complex numbers. Take easy range for Sin x and Cos x for a simple x,

Let a = Sin x, Cox = Sqrt (1 - a^2)

a + Sqrt (1 - a^2) = 2

=> 2 a^2 - 4 a + 3 = 0

Now, we want Sin 2 x and it is still 2 Sin x Cox,

2 a Sqrt (1 - a^2 ) = 2 a (2 - a) = 4 a - 2 a^2 = 4 a - 4 a + 3 = 3 {From above}

sin 2 x = 2 sin x cos x \sin 2x = 2\sin x \cos x

( sin x + cos x ) 2 = 4 (\sin x + \cos x)^2 = 4

sin 2 x + 2 sin x cos x + cos 2 x = 4 sin 2 x + cos 2 x = 1 \sin^2 x + 2\sin x\cos x + \cos^2 x = 4 \implies \boxed{\sin^2 x + \cos^2 x = 1}

2 sin x cos x = 3 sin 2 x = 3 2\sin x\cos x = 3 \implies \sin 2x = \boxed{3}

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