x → + ∞ lim x 1 − cos x = ?
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We rewrite 1 − cos x as 2 sin 2 2 x .
Observe that 2 sin 2 2 x can attain a maximum value of 2 and a minimum of zero. As x approaches infinity, this it keeps on oscillating between its lower and upper bounds.
Thus, the limit would tend to zero as x goes to infinity.
Note that 0 ≤ ∣ ∣ ∣ ∣ x 1 − cos x ∣ ∣ ∣ ∣ ≤ ∣ x ∣ 1 + ∣ cos x ∣ ≤ ∣ x ∣ 2
Now, taking x → + ∞ , we conclude by the squeeze theorem/sandwich theorem that x → + ∞ lim x 1 − cos x = 0
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− 1 ≤ cos x ≤ 1
⇒ 0 ≤ 1 − cos x ≤ 2
Since the numerator is finite for any real x we can say that:
x → + ∞ lim x 1 − cos x = k × x → + ∞ lim x 1 ; k ∈ [ 0 , 2 ]
⇒ x → + ∞ lim x 1 − cos x = k × 0 = 0