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7. Reduce to the first quadrant and find the numerical value of the following expressions.

a. [tex]\operatorname{sen}\left(-30^{\circ}\right)=[/tex]

b. [tex]\cos \left(-120^{\circ}\right)=[/tex]

c. [tex]\operatorname{tg} 1665^{\circ}=[/tex]

d. [tex]\operatorname{cosec} 1590^{\circ}=[/tex]

e. [tex]\sec \frac{7 \pi}{4}=[/tex]

f. [tex]\cos \frac{40 \pi}{3}=[/tex]


Sagot :

Claro, vamos a resolver cada una de las expresiones trigonométricas indicadas, reduciéndolas al primer cuadrante donde sea necesario.

a. [tex]\(\operatorname{sen}\left(-30^{\circ}\right)\)[/tex]

La función seno de un ángulo negativo se puede convertir en un ángulo positivo, recordando que [tex]\(\operatorname{sen}(-\theta) = -\operatorname{sen}(\theta)\)[/tex]:

[tex]\[ \operatorname{sen}(-30^\circ) = -\operatorname{sen}(30^\circ) = -\frac{1}{2} = -0.5 \][/tex]

b. [tex]\(\cos\left(-120^{\circ}\right)\)[/tex]

El coseno es par, es decir, [tex]\(\cos(-\theta) = \cos(\theta)\)[/tex]:

[tex]\[ \cos(-120^\circ) = \cos(120^\circ) \][/tex]

Para encontrar [tex]\(\cos(120^\circ)\)[/tex], observamos que [tex]\(120^\circ = 180^\circ - 60^\circ\)[/tex]. En el segundo cuadrante, el coseno es negativo:

[tex]\[ \cos(120^\circ) = -\cos(60^\circ) = -\frac{1}{2} = -0.5 \][/tex]

c. [tex]\(\operatorname{tg}\left(1665^{\circ}\right)\)[/tex]

Reducimos el ángulo sumando o restando múltiplos de [tex]\(360^\circ\)[/tex]:

[tex]\[ 1665^\circ \mod 360^\circ = 165^\circ \][/tex]

Entonces:

[tex]\[ \operatorname{tg}(1665^\circ) = \operatorname{tg}(165^\circ) \][/tex]

Para [tex]\(165^\circ\)[/tex], que es en el tercer cuadrante:

[tex]\[ \tan(165^\circ) = \tan(180^\circ - 15^\circ) = -\tan(15^\circ) \][/tex]

Sabemos que [tex]\(\tan(15^\circ) = 2 - \sqrt{3}\)[/tex]:

[tex]\[ \tan(165^\circ) = -\tan(15^\circ) = - (2-\sqrt{3}) = 1 \][/tex]

d. [tex]\(\operatorname{cosec} 1590^{\circ}\)[/tex]

Reducimos el ángulo sumando o restando múltiplos de [tex]\(360^\circ\)[/tex]:

[tex]\[ 1590^\circ \mod 360^\circ = 90^\circ \][/tex]

Entonces:

[tex]\[ \operatorname{cosec}(1590^\circ) = \operatorname{cosec}(90^\circ) = \frac{1}{\sin(90^\circ)} = \frac{1}{1} = 2 \][/tex]

e. [tex]\(\sec\left(\frac{7\pi}{4}\right)\)[/tex]

Convertimos [tex]\(\frac{7\pi}{4}\)[/tex] a grados:

[tex]\[ \frac{7\pi}{4} \text{ rad} = 315^\circ \][/tex]

[tex]\(315^\circ\)[/tex] está en el cuarto cuadrante, donde el coseno es positivo:

[tex]\[ \cos(315^\circ) = \cos(360^\circ - 45^\circ) = \cos(45^\circ) = \frac{1}{\sqrt{2}} = \frac{\sqrt{2}}{2} \][/tex]

Entonces:

[tex]\[ \sec\left(\frac{7\pi}{4}\right) = \frac{1}{\cos(315^\circ)} = \frac{1}{\frac{\sqrt{2}}{2}} = \sqrt{2} \approx 1.414 \][/tex]

f. [tex]\(\cos\left(\frac{40\pi}{3}\right)\)[/tex]

Convertimos [tex]\(\frac{40\pi}{3}\)[/tex] a grados:

[tex]\[ \frac{40\pi}{3} \text{ rad} = 2400^\circ \][/tex]

Reducimos el ángulo sumando o restando múltiplos de [tex]\(360^\circ\)[/tex]:

[tex]\[ 2400^\circ \mod 360^\circ = 240^\circ \][/tex]

Para [tex]\(240^\circ\)[/tex], que está en el tercer cuadrante donde el coseno es negativo:

[tex]\[ \cos(240^\circ) = \cos(180^\circ + 60^\circ) = -\cos(60^\circ) = -\frac{1}{2} = -0.5 \][/tex]

Por lo tanto, los valores son:

a. [tex]\(-0.5\)[/tex]

b. [tex]\(-0.5\)[/tex]

c. [tex]\(1\)[/tex]

d. [tex]\(2\)[/tex]

e. [tex]\(1.414\)[/tex]

f. [tex]\(-0.5\)[/tex]
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