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Which of the following functions are dec...

Which of the following functions are decreasing on `(0,\ pi//2)` ? (i) `cosx` (ii) `cos2x` (iii) `tanx` (iv) `cos3x`

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Given,`f(x_1)=cosx`
`f(x_2)=cos2x`
`f(x_3)=tanx`
`f(x_4)=cos3x`
Now
`f'(x_1)=-sinx<0`
...
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