1
$( ۱-{{\cos }^{۲}}\theta )( ۱+\frac{۱}{{{\cos }^{۲}}\theta } )={{\sin }^{۲}}\theta +{{\tan }^{۲}}\theta $
✓
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2
${{\sin }^{۴}}\theta -{{\cos }^{۴}}\theta =۲{{\sin }^{۲}}\theta -۱$
✓
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3
$\frac{۲\tan \theta }{۱+{{\tan }^{۲}}\theta }=۲\sin \theta \cos \theta $
✓
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4
$\frac{۱+\cos\theta }{\sin \theta }+\frac{\sin \theta }{۱+\cos\theta }=\frac{۱}{\sin \theta }$
✓
✗
$\frac{1+\cos\theta }{\sin \theta }+\frac{\sin \theta }{1+\cos\theta }=\frac{2}{\sin \theta }$