Given that sin⁡A=−35\sin A = -\frac{3}{5} and cos⁡B=−817\cos B = -\frac{8}{17}, where angle AA and angle BB are in the same quadrant, find the exact values of

  1. (i)

    tan⁡(A+B)\tan(A + B)

  2. (ii)

    cos⁡B2\cos \frac{B}{2}

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Worked solution

4(i)

1. Determine the quadrant for angles A and B

Since sin⁡A=−35\sin A = -\frac{3}{5} (negative) and cos⁡B=−817\cos B = -\frac{8}{17} (negative), and they are in the same quadrant, both AA and BB must be in the 3rd quadrant.

2. Find the exact values of tan A and tan B

In the 3rd quadrant, tangent is positive.
For angle AA: opposite = 3, hypotenuse = 5   ⟹  \implies adjacent = 52−32=4\sqrt{5^2 - 3^2} = 4.
tan⁡A=34\tan A = \frac{3}{4}

For angle BB: adjacent = 8, hypotenuse = 17   ⟹  \implies opposite = 172−82=15\sqrt{17^2 - 8^2} = 15.
tan⁡B=158\tan B = \frac{15}{8}

3. Apply the compound angle formula for tan(A + B)

tan⁡(A+B)=tan⁡A+tan⁡B1−tan⁡Atan⁡B\tan(A + B) = \frac{\tan A + \tan B}{1 - \tan A \tan B}

4. Substitute the values and evaluate

tan⁡(A+B)=34+1581−(34)(158)\tan(A + B) = \frac{\frac{3}{4} + \frac{15}{8}}{1 - \left(\frac{3}{4}\right)\left(\frac{15}{8}\right)}
=68+1581−4532= \frac{\frac{6}{8} + \frac{15}{8}}{1 - \frac{45}{32}}
=21832−4532= \frac{\frac{21}{8}}{\frac{32 - 45}{32}}
=218−1332= \frac{\frac{21}{8}}{-\frac{13}{32}}
=218×(−3213)= \frac{21}{8} \times \left(-\frac{32}{13}\right)
=−8413= -\frac{84}{13}

Answer

7736\frac{77}{36}

4(ii)

1. Determine the quadrant of angle $B$ and $\frac{B}{2}$

Since sin⁡A=−35<0\sin A = -\frac{3}{5} < 0, AA is in the 3rd or 4th quadrant.
Since cos⁡B=−817<0\cos B = -\frac{8}{17} < 0, BB is in the 2nd or 3rd quadrant.
Given that AA and BB are in the same quadrant, they must both be in the 3rd quadrant.
Therefore, 180∘<B<270∘180^\circ < B < 270^\circ.
Dividing by 2 gives 90∘<B2<135∘90^\circ < \frac{B}{2} < 135^\circ, which means B2\frac{B}{2} is in the 2nd quadrant.
In the 2nd quadrant, cos⁡B2\cos \frac{B}{2} is negative.

2. Apply the double angle formula for cosine to find $\cos^2 \frac{B}{2}$

Using the identity cos⁡B=2cos⁡2B2−1\cos B = 2\cos^2 \frac{B}{2} - 1:
−817=2cos⁡2B2−1-\frac{8}{17} = 2\cos^2 \frac{B}{2} - 1
2cos⁡2B2=1−817=9172\cos^2 \frac{B}{2} = 1 - \frac{8}{17} = \frac{9}{17}
cos⁡2B2=934\cos^2 \frac{B}{2} = \frac{9}{34}

3. Solve for $\cos \frac{B}{2}$ and select the correct sign

cos⁡B2=±934=±334\cos \frac{B}{2} = \pm \sqrt{\frac{9}{34}} = \pm \frac{3}{\sqrt{34}}
Since cos⁡B2\cos \frac{B}{2} must be negative:
cos⁡B2=−334=−33434\cos \frac{B}{2} = -\frac{3}{\sqrt{34}} = -\frac{3\sqrt{34}}{34}

Answer

−33434-\frac{3\sqrt{34}}{34}

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