15 Дек 2021 в 19:40
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To prove the inequality sinπ/3−2xπ/3 - 2xπ/32xcosπ/3−2xπ/3 - 2xπ/32x ≥ -√3/4, we can use the double angle formula for sine and cosine:

sin2θ2θ2θ = 2sinθθθcosθθθ cos2θ2θ2θ = cos^2θθθ - sin^2θθθ

Let's first substitute θ = π/6 - x into the double angle formulas:

sinπ/3−2xπ/3 - 2xπ/32x = 2sinπ/6−xπ/6 - xπ/6xcosπ/6−xπ/6 - xπ/6x cosπ/3−2xπ/3 - 2xπ/32x = cos^2π/6−xπ/6 - xπ/6x - sin^2π/6−xπ/6 - xπ/6x

Now, substitute these expressions back into the original inequality:

2sinπ/6−xπ/6 - xπ/6xcosπ/6−xπ/6 - xπ/6xcos2(π/6−x)−sin2(π/6−x)cos^2(π/6 - x) - sin^2(π/6 - x)cos2(π/6x)sin2(π/6x) ≥ -√3/4

Expand the expression and simplify:

2sinπ/6−xπ/6 - xπ/6xcosπ/6−xπ/6 - xπ/6xcos^2π/6−xπ/6 - xπ/6x - 2sin^3π/6−xπ/6 - xπ/6x ≥ -√3/4

Since sinπ/6π/6π/6 = 1/2 and cosπ/6π/6π/6 = √3/2, the inequality simplifies to:

21/2−sin3(π/6−x)1/2 - sin^3(π/6 - x)1/2sin3(π/6x)√3/2 - 2sin^3π/6−xπ/6 - xπ/6x ≥ -√3/4

√3 - 3sin^3π/6−xπ/6 - xπ/6x√3 - 2sin^3π/6−xπ/6 - xπ/6x ≥ -√3/4

√3 - 5sin^3π/6−xπ/6 - xπ/6x√3 ≥ -√3/4

1 - 5sin^3π/6−xπ/6 - xπ/6x ≥ -1/4

5sin^3π/6−xπ/6 - xπ/6x ≤ 5/4

sin^3π/6−xπ/6 - xπ/6x ≤ 1/4

Since -1 ≤ sinxxx ≤ 1, and sinπ/6−xπ/6 - xπ/6x is in the range of sinxxx, we have established that sin^3π/6−xπ/6 - xπ/6x ≤ 1/4. Thus, the inequality sinπ/3−2xπ/3 - 2xπ/32xcosπ/3−2xπ/3 - 2xπ/32x ≥ -√3/4 holds true.

16 Апр 2024 в 20:12
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