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Question Number 98521 by pranesh last updated on 14/Jun/20

Answered by abdomathmax last updated on 15/Jun/20

$$\mathrm{we}\mathrm{have}\mid \mathrm{x}\mid +\mathrm{x}⩾0\mathrm{for}\mathrm{all}\mathrm{x}$$$$\mathrm{if}\mathrm{x}⩾0\mathrm{we}\mathrm{get}\mathrm{x}+\mathrm{x}⩾0\mathrm{true}$$$$\mathrm{if}\mathrm{x}⩽0\mathrm{we}\mathrm{get}-\mathrm{x}+\mathrm{x}⩾0\mathrm{also}\mathrm{true}\mathrm{so}$$$$\mathrm{x}\in {\mathrm{D}}_{\mathrm{f}}\iff \mathrm{x}>0\mathrm{and}\mathrm{x}\ne \frac{\pi }{2}+\mathrm{k}\pi (\mathrm{k}\in \mathrm{Z})$$

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