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Geometry Level 2

In the domain [ 0 , 100 0 ] [0^\circ , 1000^\circ ] , how many solutions are there to sin θ cos θ = 1 2 \sin \theta \cos \theta = \frac {1} {2} ?


The answer is 6.

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6 solutions

Sherry Sarkar
Dec 14, 2013

The expression reminds us of sin 2 θ \sin 2\theta which is 2 × sin θ cos θ 2\times \sin \theta \cos \theta . We multiply both sides of the expression by two to come up with the neat expression of sin 2 θ = 1 \sin 2\theta = 1 . Sine is 1 when the angle is 90 or co terminal to 90. The range of 2 θ 2\theta is 0 to 2,000. Thus we have 90, 450, 810, 1170, 1530, and 1890 or 6 \boxed {6} angles.

Amlan Mishra
Dec 24, 2013

sinAcosA = 1/2 only when A = 45 deg so A = {45,225,405,585,765,945} so A has 6 values.

Tom Zhou
Dec 20, 2013

Note that sin θ cos θ = 2 sin θ cos θ 2 = sin 2 θ 2 = 1 2 \sin\theta \cos\theta=\frac{2\sin\theta\cos\theta}{2}=\frac{\sin2\theta}{2}=\frac{1}{2} . So we have sin 2 θ = 1 \sin2\theta=1 and θ = ( 2 k 1 ) π 2 \theta=\frac{(2k-1)\pi}{2} for k Z k\in\mathbb{Z} . The smallest solution that falls in the range [ 0 , 100 0 ] [0^{\circ},1000^{\circ}] is for k = 1 k=1 and the largest is for k = 6 k=6 . So there are 6 \boxed{6} solutions.

Utkarsh Verma
Dec 19, 2013

take 2 to left hand side... this makes \sin \2theta=1 hence we get \theta=pi/4 for solutions till 1000 degrees add pi to pi/4..

Jordi Bosch
Dec 15, 2013

Doing some Algebra manipulation we get:

2 sin θ cos θ = 1 sin 2 θ = 1 2\sin \theta\cos \theta = 1 \rightarrow \sin 2\theta = 1

We know that if θ = 90 , 270 \theta = 90, 270 then the equation is true.

So the general solution is 45 + 180 n 45 + 180n will be the general solution.

0 45 + 180 n < 1000 0 \le45 + 180n < 1000 Solving for nonegative integers we get:

0 n 5 0\le n \le 5

Thus, 6 \boxed{6} possibilities.

sin θ cos θ = 1 2 \sin\theta\cos\theta = \frac{1}{2} simplifies into:
2 sin θ cos θ = 1 2\sin\theta\cos\theta = 1
Using the trig identity: 2 sin θ cos θ = sin 2 θ = 1 2\sin\theta\cos\theta = \sin2\theta = 1
sin 2 θ = 1 \sin2\theta = 1 when 2 θ = 90 d e g r e e s + 360 d e g r e e s 2\theta= 90 degrees + 360 degrees
This happens when θ = 45 d e g r e e s + 180 d e g r e e s \theta= 45 degrees+180 degrees
The solutions are the numbers: 45, 225, 405, 585, 765, and 945 degrees.
This means there are 6 s o l u t i o n s . \boxed{6 solutions.}





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