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Ch. 7 - Transcendental Functions
Hass - Thomas' Calculus 15th Edition
Hass15th EditionThomas' CalculusISBN: 9780137616077Non è quello che usi tu?Cambia libro di testo
Capitolo 7, Problema 7.PE.91

Use l’Hôpital’s Rule to find the limits in Exercises 85–108.
91. lim(x→π/2⁻) (sec(7x))(cos(3x))

Guida verificata passo dopo passo
1
First, rewrite the limit expression to understand its behavior as \(x\) approaches \(\frac{\pi}{2}\) from the left: \(\lim_{x \to \frac{\pi}{2}^-} (\sec(7x)) (\cos(3x))\).
Recall that \(\sec(\theta) = \frac{1}{\cos(\theta)}\), so rewrite the expression as \(\lim_{x \to \frac{\pi}{2}^-} \frac{\cos(3x)}{\cos(7x)}\).
Evaluate the numerator and denominator separately at \(x = \frac{\pi}{2}\) to check if the limit is an indeterminate form: \(\cos(3 \cdot \frac{\pi}{2})\) and \(\cos(7 \cdot \frac{\pi}{2})\).
If direct substitution results in an indeterminate form like \(\frac{0}{0}\) or \(\frac{\infty}{\infty}\), apply l’Hôpital’s Rule by differentiating the numerator and denominator with respect to \(x\):
\[\lim_{x \to \frac{\pi}{2}^-} \frac{\frac{d}{dx} \cos(3x)}{\frac{d}{dx} \cos(7x)} = \lim_{x \to \frac{\pi}{2}^-} \frac{-3 \sin(3x)}{-7 \sin(7x)} = \lim_{x \to \frac{\pi}{2}^-} \frac{3 \sin(3x)}{7 \sin(7x)}.\]
Finally, evaluate this new limit by substituting \(x = \frac{\pi}{2}\) and simplify to find the limit.

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