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Which expression is defined as a real number?

Answer and explanation

Correct answer: \(\tan^{-1}5\)

The direct answer is option C: \(\tan^{-1}5\) is a real number. In inverse trigonometric functions, the input must belong to the function's domain. Since tangent can take every real value, \(\tan^{-1}x\) is defined for every real x; therefore 5 is allowed. Option A, \(\sin^{-1}2\), is not real because sine values lie only from -1 to 1, and 2 is outside that interval. Option B, \(\cos^{-1}(-3/2)\), is not real because cosine also has range [-1,1], while \(-3/2\) is less than -1. Option D, \(\sec^{-1}(1/2)\), is not real under the usual real inverse-secant definition because secant values have absolute value at least 1; \(|1/2|<1\). Thus C alone is defined in the reals. Memory cue: check the input range before using an inverse trigonometric function.

Related tags

Inverse-TrigonometryDomain-CheckConcept

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