Find all values of xx which satisfies the equation (2+3cos⁡x)(4−5sin⁡x)=0(2 + 3 \cos x)(4 - 5 \sin x) = 0 for 0≤x≤70 \le x \le 7.

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Worked solution

4

1. Set each factor to zero to find the basic trigonometric equations

Since (2+3cos⁡x)(4−5sin⁡x)=0(2 + 3 \cos x)(4 - 5 \sin x) = 0, we have:
2+3cos⁡x=0  ⟹  cos⁡x=−232 + 3 \cos x = 0 \implies \cos x = -\frac{2}{3}
or
4−5sin⁡x=0  ⟹  sin⁡x=454 - 5 \sin x = 0 \implies \sin x = \frac{4}{5}

2. Solve $\cos x = -\frac{2}{3}$ for $0 \le x \le 7$

Basic angle α=cos⁡−1(23)≈0.84107\alpha = \cos^{-1}\left(\frac{2}{3}\right) \approx 0.84107 rad.
Since cos⁡x\cos x is negative, xx is in the 2nd and 3rd quadrants.
x=π−0.84107 or π+0.84107x = \pi - 0.84107 \text{ or } \pi + 0.84107
x≈2.3005 or 3.9827x \approx 2.3005 \text{ or } 3.9827
(The next solution 3π−0.84107≈8.58>73\pi - 0.84107 \approx 8.58 > 7)

3. Solve $\sin x = \frac{4}{5}$ for $0 \le x \le 7$

Basic angle β=sin⁡−1(45)≈0.92730\beta = \sin^{-1}\left(\frac{4}{5}\right) \approx 0.92730 rad.
Since sin⁡x\sin x is positive, xx is in the 1st and 2nd quadrants.
x=0.92730 or π−0.92730x = 0.92730 \text{ or } \pi - 0.92730
x≈0.92730 or 2.2143x \approx 0.92730 \text{ or } 2.2143
(The next solution 2π+0.92730≈7.21>72\pi + 0.92730 \approx 7.21 > 7)

4. State the final values of $x$ rounded to 3 significant figures

x=0.927,2.21,2.30,3.98x = 0.927, 2.21, 2.30, 3.98

Answer

x=0.927,2.21,2.30,3.98x = 0.927, 2.21, 2.30, 3.98

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