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Let f(x)=xsinpix ,\ x >0 . Then for ...

Let `f(x)=xsinpix ,\ x >0` . Then for all natural numbers `n ,\ f^(prime)(x)` vanishes at (a) A unique point in the interval `(n ,\ n+1/2)` (b) a unique point in the interval `(n+1/2,\ n+1)` (c) a unique point in the interval `(n ,\ n+1)` (d) two points in the interval `(n ,\ n+1)`

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We have `f'(x)=sinpix+pixcospix=0`
`implies" "tanpix=-pix`
Now we draw the graphs of `y=tanpix` and `y=-pix` and look for their points of intersection. `y=tanpix` has period 1.
Graphs are as shown in the following figure.

`implies" " x in (n + (1)/(2), n + 1) or (n, n + 1)`
Hence (b) and (c) are correct options.
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CENGAGE PUBLICATION-GRAPHS OF TRIGONOMETRIC FUNCTIONS-Exercises
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