Minimum Value

Calculus Level 2

f ( x ) = 3 sin x + 4 cos x + 2014 f(x) = 3\sin x + 4\cos x + 2014

Find the minimum value of f ( x ) f(x) .


The answer is 2009.

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2 solutions

Sanjeet Raria
Sep 25, 2014

Note that, a 2 + b 2 a sin θ + b cos θ a 2 + b 2 -\sqrt{a^2+b^2}\leq a\sin\theta+b\cos\theta\leq \sqrt{a^2+b^2} Hence minimum value of the function is 5 + 2014 = 2009 -5+2014=\boxed{2009}

Note first that

3 sin ( x ) + 4 cos ( x ) = 5 ( ( 3 5 ) sin ( x ) + ( 4 5 ) cos ( x ) ) = 5 sin ( x + arccos ( 3 5 ) ) 3\sin(x) + 4\cos(x) = 5*((\frac{3}{5})\sin(x) + (\frac{4}{5})\cos(x)) = 5*\sin(x + \arccos(\frac{3}{5})) .

Thus, since the minimum of sin ( θ ) \sin(\theta) is 1 -1 , the minimum of 3 sin ( x ) + 4 cos ( x ) 3\sin(x) + 4\cos(x) is 5 ( 1 ) = 5 5*(-1) = -5 , and thus the minimum of f ( x ) f(x) is ( 5 ) + 2014 = 2009 (-5) + 2014 = \boxed{2009} .

(Alternately, we have that f ( x ) = 0 f'(x) = 0 when tan ( x ) = 3 4 \tan(x) = \frac{3}{4} , which leads to minimum co-values for sin ( x ) \sin(x) and cos ( x ) \cos(x) of 3 5 -\frac{3}{5} and 4 5 -\frac{4}{5} , respectively. This results in the same minimum value for f ( x ) f(x) as found above.)

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