Given that sin⁡(A+B)sin⁡(A−B)=15\frac{\sin(A + B)}{\sin(A - B)} = \frac{1}{5}, prove that 2tan⁡A+3tan⁡B=02\tan A + 3\tan B = 0. Hence, solve the equation sin⁡(x−45∘)=5sin⁡(x+45∘)\sin(x - 45^\circ) = 5\sin(x + 45^\circ) for 0∘<x<360∘0^\circ < x < 360^\circ.

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Worked solution

10(i)

1. Cross-multiply the given equation to remove fractions.

5sin⁡(A+B)=sin⁡(A−B)5\sin(A + B) = \sin(A - B)

2. Expand both sides using the addition formulae for sine.

5(sin⁡Acos⁡B+cos⁡Asin⁡B)=sin⁡Acos⁡B−cos⁡Asin⁡B5(\sin A \cos B + \cos A \sin B) = \sin A \cos B - \cos A \sin B

3. Expand the brackets and collect like terms.

5sin⁡Acos⁡B+5cos⁡Asin⁡B=sin⁡Acos⁡B−cos⁡Asin⁡B5\sin A \cos B + 5\cos A \sin B = \sin A \cos B - \cos A \sin B
4sin⁡Acos⁡B+6cos⁡Asin⁡B=04\sin A \cos B + 6\cos A \sin B = 0

4. Divide the equation by $\cos A \cos B$ to express in terms of tangent, and simplify.

4sin⁡Acos⁡Bcos⁡Acos⁡B+6cos⁡Asin⁡Bcos⁡Acos⁡B=0\frac{4\sin A \cos B}{\cos A \cos B} + \frac{6\cos A \sin B}{\cos A \cos B} = 0
4tan⁡A+6tan⁡B=04\tan A + 6\tan B = 0
2tan⁡A+3tan⁡B=0(Shown)2\tan A + 3\tan B = 0 \quad \text{(Shown)}

Answer

Shown.

10(ii)

1. Rearrange the given equation to match the form in part (i)

sin⁡(x−45∘)=5sin⁡(x+45∘)\sin(x - 45^\circ) = 5\sin(x + 45^\circ)
sin⁡(x+45∘)sin⁡(x−45∘)=15\frac{\sin(x + 45^\circ)}{\sin(x - 45^\circ)} = \frac{1}{5}

2. Apply the result from part (i) to form an equation in terms of tan

Using the result from (i) with A=xA = x and B=45∘B = 45^\circ:
2tan⁡x+3tan⁡45∘=02\tan x + 3\tan 45^\circ = 0

3. Evaluate $\tan 45^\circ$ and solve for $\tan x$

Since tan⁡45∘=1\tan 45^\circ = 1:
2tan⁡x+3(1)=02\tan x + 3(1) = 0
tan⁡x=−32=−1.5\tan x = -\frac{3}{2} = -1.5

4. Find the basic angle and solve for $x$ in the given domain

Basic angle α=tan⁡−1(1.5)≈56.3099∘\alpha = \tan^{-1}(1.5) \approx 56.3099^\circ
Since tan⁡x\tan x is negative, xx lies in the 2nd and 4th quadrants.
x=180∘−56.3099∘=123.6901∘≈123.7∘x = 180^\circ - 56.3099^\circ = 123.6901^\circ \approx 123.7^\circ
x=360∘−56.3099∘=303.6901∘≈303.7∘x = 360^\circ - 56.3099^\circ = 303.6901^\circ \approx 303.7^\circ

Answer

x=123.7∘,303.7∘x = 123.7^\circ, 303.7^\circ

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