Question 1011910
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The non-permissible values would cause 
division by zero (undefined), so:
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{{{cos^2(x)-1=0}}} is not permitted.
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{{{cos^2(x)-1=0}}}
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{{{cos^2(x)=1}}}
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{{{sqrt(cos^2(x))=sqrt(1)}}}
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{{{cos(x)}}}=+-{{{(1)}}}
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{{{arccos(1)=0}}} {{{and}}} {{{arccos(-1)=180}}}
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ANSWER: Non-permissible values are x=0 
degrees or x=180 degrees.