Se da la desigualdad:
( 2 sin 2 ( x ) + 3 sin ( x ) cos ( x ) ) − 3 cos 2 ( x ) > 1 \left(2 \sin^{2}{\left(x \right)} + 3 \sin{\left(x \right)} \cos{\left(x \right)}\right) - 3 \cos^{2}{\left(x \right)} > 1 ( 2 sin 2 ( x ) + 3 sin ( x ) cos ( x ) ) − 3 cos 2 ( x ) > 1 Para resolver esta desigualdad primero hay que resolver la ecuación correspondiente:
( 2 sin 2 ( x ) + 3 sin ( x ) cos ( x ) ) − 3 cos 2 ( x ) = 1 \left(2 \sin^{2}{\left(x \right)} + 3 \sin{\left(x \right)} \cos{\left(x \right)}\right) - 3 \cos^{2}{\left(x \right)} = 1 ( 2 sin 2 ( x ) + 3 sin ( x ) cos ( x ) ) − 3 cos 2 ( x ) = 1 Resolvemos:
x 1 = − 3 π 4 x_{1} = - \frac{3 \pi}{4} x 1 = − 4 3 π x 2 = π 4 x_{2} = \frac{\pi}{4} x 2 = 4 π x 3 = 2 atan ( 1 4 − 17 4 ) x_{3} = 2 \operatorname{atan}{\left(\frac{1}{4} - \frac{\sqrt{17}}{4} \right)} x 3 = 2 atan ( 4 1 − 4 17 ) x 4 = 2 atan ( 1 4 + 17 4 ) x_{4} = 2 \operatorname{atan}{\left(\frac{1}{4} + \frac{\sqrt{17}}{4} \right)} x 4 = 2 atan ( 4 1 + 4 17 ) x 1 = − 3 π 4 x_{1} = - \frac{3 \pi}{4} x 1 = − 4 3 π x 2 = π 4 x_{2} = \frac{\pi}{4} x 2 = 4 π x 3 = 2 atan ( 1 4 − 17 4 ) x_{3} = 2 \operatorname{atan}{\left(\frac{1}{4} - \frac{\sqrt{17}}{4} \right)} x 3 = 2 atan ( 4 1 − 4 17 ) x 4 = 2 atan ( 1 4 + 17 4 ) x_{4} = 2 \operatorname{atan}{\left(\frac{1}{4} + \frac{\sqrt{17}}{4} \right)} x 4 = 2 atan ( 4 1 + 4 17 ) Las raíces dadas
x 1 = − 3 π 4 x_{1} = - \frac{3 \pi}{4} x 1 = − 4 3 π x 3 = 2 atan ( 1 4 − 17 4 ) x_{3} = 2 \operatorname{atan}{\left(\frac{1}{4} - \frac{\sqrt{17}}{4} \right)} x 3 = 2 atan ( 4 1 − 4 17 ) x 2 = π 4 x_{2} = \frac{\pi}{4} x 2 = 4 π x 4 = 2 atan ( 1 4 + 17 4 ) x_{4} = 2 \operatorname{atan}{\left(\frac{1}{4} + \frac{\sqrt{17}}{4} \right)} x 4 = 2 atan ( 4 1 + 4 17 ) son puntos de cambio del signo de desigualdad en las soluciones.
Primero definámonos con el signo hasta el punto extremo izquierdo:
x 0 < x 1 x_{0} < x_{1} x 0 < x 1 Consideremos, por ejemplo, el punto
x 0 = x 1 − 1 10 x_{0} = x_{1} - \frac{1}{10} x 0 = x 1 − 10 1 =
− 3 π 4 − 1 10 - \frac{3 \pi}{4} - \frac{1}{10} − 4 3 π − 10 1 =
− 3 π 4 − 1 10 - \frac{3 \pi}{4} - \frac{1}{10} − 4 3 π − 10 1 lo sustituimos en la expresión
( 2 sin 2 ( x ) + 3 sin ( x ) cos ( x ) ) − 3 cos 2 ( x ) > 1 \left(2 \sin^{2}{\left(x \right)} + 3 \sin{\left(x \right)} \cos{\left(x \right)}\right) - 3 \cos^{2}{\left(x \right)} > 1 ( 2 sin 2 ( x ) + 3 sin ( x ) cos ( x ) ) − 3 cos 2 ( x ) > 1 − 3 cos 2 ( − 3 π 4 − 1 10 ) + ( 2 sin 2 ( − 3 π 4 − 1 10 ) + 3 sin ( − 3 π 4 − 1 10 ) cos ( − 3 π 4 − 1 10 ) ) > 1 - 3 \cos^{2}{\left(- \frac{3 \pi}{4} - \frac{1}{10} \right)} + \left(2 \sin^{2}{\left(- \frac{3 \pi}{4} - \frac{1}{10} \right)} + 3 \sin{\left(- \frac{3 \pi}{4} - \frac{1}{10} \right)} \cos{\left(- \frac{3 \pi}{4} - \frac{1}{10} \right)}\right) > 1 − 3 cos 2 ( − 4 3 π − 10 1 ) + ( 2 sin 2 ( − 4 3 π − 10 1 ) + 3 sin ( − 4 3 π − 10 1 ) cos ( − 4 3 π − 10 1 ) ) > 1 2/1 pi\ 2/1 pi\ /1 pi\ /1 pi\
- 3*sin |-- + --| + 2*cos |-- + --| + 3*cos|-- + --|*sin|-- + --| > 1
\10 4 / \10 4 / \10 4 / \10 4 / Entonces
x < − 3 π 4 x < - \frac{3 \pi}{4} x < − 4 3 π no se cumple
significa que una de las soluciones de nuestra ecuación será con:
x > − 3 π 4 ∧ x < 2 atan ( 1 4 − 17 4 ) x > - \frac{3 \pi}{4} \wedge x < 2 \operatorname{atan}{\left(\frac{1}{4} - \frac{\sqrt{17}}{4} \right)} x > − 4 3 π ∧ x < 2 atan ( 4 1 − 4 17 ) _____ _____
/ \ / \
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x1 x3 x2 x4 Recibiremos otras soluciones de la desigualdad pasando al polo siguiente etc.
etc.
Respuesta:
x > − 3 π 4 ∧ x < 2 atan ( 1 4 − 17 4 ) x > - \frac{3 \pi}{4} \wedge x < 2 \operatorname{atan}{\left(\frac{1}{4} - \frac{\sqrt{17}}{4} \right)} x > − 4 3 π ∧ x < 2 atan ( 4 1 − 4 17 ) x > π 4 ∧ x < 2 atan ( 1 4 + 17 4 ) x > \frac{\pi}{4} \wedge x < 2 \operatorname{atan}{\left(\frac{1}{4} + \frac{\sqrt{17}}{4} \right)} x > 4 π ∧ x < 2 atan ( 4 1 + 4 17 )