Prove cos x < ( sin x)/x < 1 by sandwich...

Prove `cos x < ( sin x)/x < 1` by sandwich theorem

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`f(x)=sinx-xcosx`
`f'(x)=cosx-cosx+xsinx`
`=xsinx`
for x`in (0,pi/2)`
`f'(x)>0`
`f(x)>0`
sinx>xcosx
`sinx/x>cosx-(1)`
...

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