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Prove that the function f(x)=cosx is str...

Prove that the function `f(x)=cosx` is strictly decreasing in `(0,\ pi)`

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We have given
`f(x)=cosx`
`f′(x)=−sinx`
As ,
`x in(0,pi)`
`impliessinx`>`0`
`implies−sinx`<`0`
`impliesf′(x)`<`0`
...
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