A number or a short expression. Spacing and $ signs are ignored.
Solution
To solve the given problem, we proceed as follows:
First, we express 2cos10∘ in terms of sine using the co-function identity, which states that sin(90∘−θ)=cosθ and vice versa. Thus, we have: 2cos10∘=2sin(90∘−10∘)=2sin80∘.
Next, we use the sine addition formula, sin(a+b)=sinacosb+cosasinb, to expand sin80∘ as sin(60∘+20∘): 2sin80∘=2sin(60∘+20∘)=2(sin60∘cos20∘+cos60∘sin20∘).
Substituting the exact values for sin60∘=23 and cos60∘=21, we get: 2(sin60∘cos20∘+cos60∘sin20∘)=2(23cos20∘+21sin20∘)=3cos20∘+sin20∘.
Therefore, substituting this result into the original expression, we have: cos20°2cos10°−sin20°=cos20°3cos20°+sin20°−sin20°=cos20°3cos20°=3.
Thus, the value of the given expression is 3.
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Source: NuminaMath-1.5,
licensed Apache-2.0.
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