Find the sum of all θ between 0 ∘ and 3 6 0 ∘ such that
cos ( 2 1 θ + 3 7 ∘ ) = 2 1 .
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same solution, @Victor Loh i donot think this is a level 3 problem rating gone boom.....
Since it is c o s ( 2 1 θ + 3 7 ° ) , the graph is 2 times longer than c o s ( x ) and shifted 3 7 ° to the left.
The possible values of x for c o s ( x ) = 2 1 is 4 5 ° and 3 1 5 ° if x is between 0 ° and 3 6 0 °
Shifting c o s ( x ) to the left by 3 7 ° would give a graph of c o s ( x + 3 7 ° ) , which the solution for c o s ( x + 3 7 ° ) = 2 1 is 4 5 ° − 3 7 ° = 8 ° and 3 1 5 ° − 3 7 ° = 2 7 8 ° .
Now, making it 2 times longer would give the graph of c o s ( 2 1 x + 3 7 ° ) , so the solution for this is 8 ° ∗ 2 = 1 6 ° therefore, the answer is 1 6
Note that 2 7 8 ° ∗ 2 = 5 5 6 ° is not counted as it is more than 3 6 0 °
The graph is now × 2 longer than before so we have to minus 3 6 0 × 2 instead of 3 6 0 , which would give a negative value and thus isn't counted.
So then we'll substract 360 from 556 ???
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We know that, cos 4 5 = 2 1 o r cos 3 1 5 = 2 1 ⇒ 2 1 θ + 3 7 = 4 5 o r 2 1 θ + 3 7 = 3 1 5 Solving both the equations for θ , we get θ = 1 6 as the only value such that 0 < θ < 3 6 0