1. Express the equation in terms of sine and cosine
3cosA+cosA3+cosA7sinA=0
Multiply throughout by cosA (assuming cosA=0):
3cos2A+3+7sinA=0
2. Convert to a quadratic equation in terms of $\sin A$
Using the identity cos2A=1−sin2A:
3(1−sin2A)+3+7sinA=0
3−3sin2A+3+7sinA=0
3sin2A−7sinA−6=0
3. Solve the quadratic equation for $\sin A$
(3sinA+2)(sinA−3)=0
sinA=−32orsinA=3
Reject sinA=3 since −1⩽sinA⩽1.
4. Find the basic angle and determine the quadrants
Basic angle α=sin−1(32)≈41.810∘
Since sinA<0, A lies in the 3rd and 4th quadrants.
5. Calculate the final values of $A$
A=180∘+41.810∘=221.810∘
A=360∘−41.810∘=318.190∘
Rounding to 1 decimal place:
A=221.8∘,318.2∘
AnswerA=221.8∘,318.2∘