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Algebra Difficulty 3.2 AMC 10/12 Find the answer

The value of 2cos10°sin20°cos20°\frac{2\cos10°-\sin20°}{\cos20°} is ____.

A number or a short expression. Spacing and $ signs are ignored.

Solution

To solve the given problem, we proceed as follows:

First, we express 2cos102\cos 10^{\circ} in terms of sine using the co-function identity, which states that sin(90θ)=cosθ\sin(90^{\circ}-\theta) = \cos\theta and vice versa. Thus, we have:
2cos10=2sin(9010)=2sin80.2\cos 10^{\circ} = 2\sin(90^{\circ}-10^{\circ}) = 2\sin 80^{\circ}.

Next, we use the sine addition formula, sin(a+b)=sinacosb+cosasinb\sin(a + b) = \sin a \cos b + \cos a \sin b, to expand sin80\sin 80^{\circ} as sin(60+20)\sin(60^{\circ}+20^{\circ}):
2sin80=2sin(60+20)=2(sin60cos20+cos60sin20).2\sin 80^{\circ} = 2\sin(60^{\circ}+20^{\circ}) = 2\left(\sin 60^{\circ}\cos 20^{\circ} + \cos 60^{\circ}\sin 20^{\circ}\right).

Substituting the exact values for sin60=32\sin 60^{\circ} = \frac{\sqrt{3}}{2} and cos60=12\cos 60^{\circ} = \frac{1}{2}, we get:
2(sin60cos20+cos60sin20)=2(32cos20+12sin20)=3cos20+sin20.2\left(\sin 60^{\circ}\cos 20^{\circ} + \cos 60^{\circ}\sin 20^{\circ}\right) = 2\left(\frac{\sqrt{3}}{2}\cos 20^{\circ} + \frac{1}{2}\sin 20^{\circ}\right) = \sqrt{3}\cos 20^{\circ} + \sin 20^{\circ}.

Therefore, substituting this result into the original expression, we have:
2cos10°sin20°cos20°=3cos20°+sin20°sin20°cos20°=3cos20°cos20°=3.\frac{2\cos10°-\sin20°}{\cos20°} = \frac{\sqrt{3}\cos20°+\sin20°-\sin20°}{\cos20°} = \frac{\sqrt{3}\cos20°}{\cos20°} = \sqrt{3}.

Thus, the value of the given expression is 3\boxed{\sqrt{3}}.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.