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Ch. 3 - Trigonometric Identities and Equations
Blitzer - Trigonometry 3rd Edition
Blitzer3rd EditionTrigonometryISBN: 9780137316601Non è quello che usi tu?Cambia libro di testo
Capitolo 3, Problema 87

In Exercises 85–96, use a calculator to solve each equation, correct to four decimal places, on the interval [0, 2𝝅). cos x = ﹣ 2/5

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Rewrite the equation clearly: \(\cos^2 x = -\frac{2}{5}\). This means the square of cosine of \(x\) equals a negative number.
Recall that \(\cos^2 x\) represents the square of the cosine function, which is always greater than or equal to zero for all real \(x\) because squaring any real number cannot produce a negative result.
Since \(\cos^2 x\) cannot be negative, analyze the equation \(\cos^2 x = -\frac{2}{5}\) and recognize that it has no real solutions because the right side is negative.
Conclude that there are no values of \(x\) in the interval \([0, 2\pi)\) that satisfy the equation \(\cos^2 x = -\frac{2}{5}\).
Therefore, the solution set is empty; no solutions exist for this equation within the given interval.

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