Solve the equation 3cos⁡A+3sec⁡A+7tan⁡A=03\cos A + 3\sec A + 7\tan A = 0 for 0∘<A<360∘0^\circ < A < 360^\circ.

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Worked solution

2

1. Express the equation in terms of sine and cosine

3cos⁡A+3cos⁡A+7sin⁡Acos⁡A=03\cos A + \frac{3}{\cos A} + \frac{7\sin A}{\cos A} = 0
Multiply throughout by cos⁡A\cos A (assuming cos⁡A≠0\cos A \neq 0):
3cos⁡2A+3+7sin⁡A=03\cos^2 A + 3 + 7\sin A = 0

2. Convert to a quadratic equation in terms of $\sin A$

Using the identity cos⁡2A=1−sin⁡2A\cos^2 A = 1 - \sin^2 A:
3(1−sin⁡2A)+3+7sin⁡A=03(1 - \sin^2 A) + 3 + 7\sin A = 0
3−3sin⁡2A+3+7sin⁡A=03 - 3\sin^2 A + 3 + 7\sin A = 0
3sin⁡2A−7sin⁡A−6=03\sin^2 A - 7\sin A - 6 = 0

3. Solve the quadratic equation for $\sin A$

(3sin⁡A+2)(sin⁡A−3)=0(3\sin A + 2)(\sin A - 3) = 0
sin⁡A=−23orsin⁡A=3\sin A = -\frac{2}{3} \quad \text{or} \quad \sin A = 3
Reject sin⁡A=3\sin A = 3 since −1⩽sin⁡A⩽1-1 \leqslant \sin A \leqslant 1.

4. Find the basic angle and determine the quadrants

Basic angle α=sin⁡−1(23)≈41.810∘\alpha = \sin^{-1}\left(\frac{2}{3}\right) \approx 41.810^\circ
Since sin⁡A<0\sin A < 0, AA lies in the 3rd and 4th quadrants.

5. Calculate the final values of $A$

A=180∘+41.810∘=221.810∘A = 180^\circ + 41.810^\circ = 221.810^\circ
A=360∘−41.810∘=318.190∘A = 360^\circ - 41.810^\circ = 318.190^\circ
Rounding to 1 decimal place:
A=221.8∘,318.2∘A = 221.8^\circ, 318.2^\circ

Answer

A=221.8∘,318.2∘A = 221.8^\circ, 318.2^\circ

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