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At x=(5pi)/6,\ \ f(x)=2sin3x+3cos3x is (...

At `x=(5pi)/6,\ \ f(x)=2sin3x+3cos3x` is (a) 0 (b) maximum (c) minimum (d) none of these

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we have` f(x)=2 \sin 3 x+3 \cos 3 x`
`\therefore f^{\prime}(x)=2 \cos 3 x 3+3(-\sin 3 x) \cdot 3 `
`\Rightarrow f^{\prime}(x)=6 \cos 3 x-9 \sin 3 x \ldots . . \text { (i) }`
Now` f^{\prime \prime}(x)=-18 \sin 3 x-27 \cos 3 x` ...
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