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Exercise 1.3 · Q7

Q.Find the largest possible domain of the real valued function f(x)=4−x2x2−9f(x)=\dfrac{\sqrt{4-x^2}}{\sqrt{x^2-9}}.

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Step 1. For the numerator 4−x2\sqrt{4-x^2} to be real, need 4−x2≥0⇒x2≤4⇒x∈[−2,2]4-x^2\ge0\Rightarrow x^2\le4\Rightarrow x\in[-2,2].

Step 2. For the denominator x2−9\sqrt{x^2-9} to be real AND nonzero (it can't be zero, since it's a denominator), need x2−9>0⇒x2>9⇒x∈(−∞,−3)∪(3,∞)x^2-9>0\Rightarrow x^2>9\Rightarrow x\in(-\infty,-3)\cup(3,\infty).

Step 3. The domain of ff is the intersection of both requirements: [−2,2]∩((−∞,−3)∪(3,∞))[-2,2]\cap\big((-\infty,-3)\cup(3,\infty)\big). Since [−2,2][-2,2] lies entirely between −3-3 and 33, it shares no point with x<−3x<-3 or x>3x>3. The intersection is empty. …

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