The equation xsin⁡x+cos⁡x=1.015x\sin x + \cos x = 1.015 has a positive root α\alpha close to zero. Use small-angle approximations for sin⁡x\sin x and cos⁡x\cos x to obtain an approximation to α\alpha. Give your answer correct to 3 significant figures.

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Worked solution

Q6

1. Substitute the small-angle approximations for $\sin x$ and $\cos x$ into the given equation.

Using sin⁡x≈x\sin x \approx x and cos⁡x≈1−12x2\cos x \approx 1 - \frac{1}{2}x^2 for small xx (in radians):
x(x)+(1−12x2)≈1.015x(x) + \left(1 - \frac{1}{2}x^2\right) \approx 1.015

2. Simplify the resulting algebraic expression to isolate $x^2$.

x2+1−0.5x2=1.015x^2 + 1 - 0.5x^2 = 1.015
0.5x2+1=1.0150.5x^2 + 1 = 1.015
0.5x2=0.0150.5x^2 = 0.015

3. Solve for the positive root and round to 3 significant figures.

x2=0.0150.5=0.03x^2 = \frac{0.015}{0.5} = 0.03
Since α\alpha is a positive root, we take the positive square root:
x=0.03≈0.173205x = \sqrt{0.03} \approx 0.173205
α=0.173 (to 3 s.f.)\alpha = 0.173 \text{ (to 3 s.f.)}

Answer

α=0.173\alpha = 0.173

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