SAT1000 - P321

Geometry Level pending

Given the function f ( x ) = sin ( ω x + π 3 ) ( ω > 0 ) f(x)=\sin(\omega x + \dfrac{\pi}{3})\ (\omega>0) , f ( π 6 ) = f ( π 3 ) f(\dfrac{\pi}{6})=f(\dfrac{\pi}{3}) .

If f ( x ) f(x) has minimum value but no maximum value on the interval ( π 6 , π 3 ) (\dfrac{\pi}{6},\dfrac{\pi}{3}) , then find the value of ω \omega .

If ω = a b \omega=\dfrac{a}{b} , where a , b a,b are positive coprime integers. submit a + b a+b .


Have a look at my problem set: SAT 1000 problems


The answer is 17.

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2 solutions

David Vreken
Jun 22, 2020

The period 2 π ω \frac{2\pi}{\omega} must be larger than the interval π 3 π 6 = π 6 \frac{\pi}{3} - \frac{\pi}{6} = \frac{\pi}{6} , otherwise the interval will include a maximum value, so 2 π ω > π 6 \frac{2\pi}{\omega} > \frac{\pi}{6} , which solves to ω < 12 \omega < 12 .

By symmetry the minimum will be exactly between π 6 \frac{\pi}{6} and π 3 \frac{\pi}{3} , which is at 1 2 ( π 6 + π 3 ) = π 4 \frac{1}{2}(\frac{\pi}{6} + \frac{\pi}{3}) = \frac{\pi}{4} , so sin ( ω f r a c π 4 + π 3 ) = 1 \sin (\omega frac{\pi}{4} + \frac{\pi}{3}) = -1 , which solves to ω = 14 3 \omega = \frac{14}{3} for 0 < ω < 12 0 < \omega < 12 .

Therefore, a = 14 a = 14 , b = 3 b = 3 , and a + b = 17 a + b = \boxed{17} .

The sine function is symmetric about it's minimum between two successive zeros. So ω π 6 + π 3 + ω π 3 + π 3 = 2 × 3 π 2 ω = 14 3 \omega \frac{π}{6}+\frac{π}{3}+\omega \frac{π}{3}+\frac{π}{3}=2\times \frac{3π}{2}\implies \omega =\frac{14}{3}

So, a = 14 , b = 3 a=14,b=3 and a + b = 17 a+b=\boxed {17} .

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