f ( x ) = 3 sin x + 4 cos x + 2 0 1 4
Find the minimum value of f ( x ) .
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Note first that
3 sin ( x ) + 4 cos ( x ) = 5 ∗ ( ( 5 3 ) sin ( x ) + ( 5 4 ) cos ( x ) ) = 5 ∗ sin ( x + arccos ( 5 3 ) ) .
Thus, since the minimum of sin ( θ ) is − 1 , the minimum of 3 sin ( x ) + 4 cos ( x ) is 5 ∗ ( − 1 ) = − 5 , and thus the minimum of f ( x ) is ( − 5 ) + 2 0 1 4 = 2 0 0 9 .
(Alternately, we have that f ′ ( x ) = 0 when tan ( x ) = 4 3 , which leads to minimum co-values for sin ( x ) and cos ( x ) of − 5 3 and − 5 4 , respectively. This results in the same minimum value for f ( x ) as found above.)
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Note that, − a 2 + b 2 ≤ a sin θ + b cos θ ≤ a 2 + b 2 Hence minimum value of the function is − 5 + 2 0 1 4 = 2 0 0 9