Trigonometry #2

Geometry Level 3

Find the sum of all θ \theta between 0 0^{\circ} and 36 0 360^{\circ} such that

cos ( 1 2 θ + 3 7 ) = 1 2 . \cos(\frac{1}{2}\theta+37^{\circ})=\frac{1}{\sqrt{2}}.


The answer is 16.

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2 solutions

Kshitij Johary
Jul 16, 2014

We know that, cos 45 = 1 2 o r cos 315 = 1 2 \cos { 45 } =\frac { 1 }{ \sqrt { 2 } } \quad or\quad \cos { 315 } =\frac { 1 }{ \sqrt { 2 } } 1 2 θ + 37 = 45 o r 1 2 θ + 37 = 315 \Rightarrow \frac { 1 }{ 2 } \theta +37=45\quad or\quad \frac { 1 }{ 2 } \theta +37=315 Solving both the equations for θ \theta , we get θ = 16 \boxed { \theta =16 } as the only value such that 0 < θ < 360 0<\theta <360

same solution, @Victor Loh i donot think this is a level 3 problem rating gone boom.....

Mardokay Mosazghi - 6 years, 9 months ago
Julian Poon
Jul 12, 2014

Since it is c o s ( 1 2 θ + 37 ° ) cos(\frac { 1 }{ 2 } \theta +37°) , the graph is 2 2 times longer than c o s ( x ) cos(x) and shifted 37 ° 37° to the left.

The possible values of x x for c o s ( x ) = 1 2 cos(x)=\frac { 1 }{ \sqrt { 2 } } is 45 ° 45° and 315 ° 315° if x x is between 0 ° and 360 ° 360°

Shifting c o s ( x ) cos(x) to the left by 37 ° 37° would give a graph of c o s ( x + 37 ° ) cos(x+37°) , which the solution for c o s ( x + 37 ° ) = 1 2 cos(x+37°)=\frac { 1 }{ \sqrt { 2 } } is 45 ° 37 ° = 8 ° 45°-37°=8° and 315 ° 37 ° = 278 ° 315°-37°=278° .

Now, making it 2 times longer would give the graph of c o s ( 1 2 x + 37 ° ) cos(\frac { 1 }{ 2 } x+37°) , so the solution for this is 8 ° 2 = 16 ° 8°*2=16° therefore, the answer is 16 \boxed { 16 }

Note that 278 ° 2 = 556 ° 278°*2=556° is not counted as it is more than 360 ° 360°

The graph is now × 2 \times2 longer than before so we have to minus 360 × 2 360\times2 instead of 360 360 , which would give a negative value and thus isn't counted.

So then we'll substract 360 from 556 ???

Rituraj Belhekar - 6 years, 10 months ago

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