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Ch. 7 - Applications of Trigonometry and Vectors
Lial - Trigonometry 12th Edition
Lial12th EditionTrigonometryISBN: 9780136552161당신이 사용하는 게 아니라요?교과서 변경
8장, 문제 17

Find the unknown angles in triangle ABC for each triangle that exists.


B = 74.3°, a = 859 m, b = 783 m

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1
Identify the given elements: angle \(B = 74.3^\circ\), side \(a = 859\) m (opposite angle \(A\)), and side \(b = 783\) m (opposite angle \(B\)). We need to find the unknown angles \(A\) and \(C\) in triangle \(ABC\).
Use the Law of Sines, which states: \(\frac{a}{\sin A} = \frac{b}{\sin B}\). Substitute the known values to set up the equation: \(\frac{859}{\sin A} = \frac{783}{\sin 74.3^\circ}\).
Solve for \(\sin A\) by rearranging the equation: \(\sin A = \frac{859 \times \sin 74.3^\circ}{783}\). Calculate the right-hand side to find the value of \(\sin A\) (do not compute the final value here).
Determine the possible values of angle \(A\) by taking the inverse sine (arcsin) of \(\sin A\). Remember that the sine function is positive in the first and second quadrants, so there may be two possible angles for \(A\): one acute and one obtuse (if valid).
For each possible value of \(A\), find angle \(C\) using the triangle angle sum property: \(C = 180^\circ - A - B\). Check if the resulting triangle is valid (all angles positive and sum to 180°). This will give you all possible triangles that satisfy the given conditions.

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주요 개념

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Law of Sines

The Law of Sines relates the sides and angles of a triangle through the ratio sin(A)/a = sin(B)/b = sin(C)/c. It is essential for finding unknown angles or sides when given two sides and an angle not included between them, as in this problem.
추천 영상:
4:27
Intro to Law of Sines

Triangle Angle Sum Property

The sum of the interior angles in any triangle is always 180°. After finding one unknown angle using the Law of Sines, this property helps determine the remaining angle by subtracting the known angles from 180°.
추천 영상:
4:47
Sum and Difference of Tangent

Ambiguous Case of the Law of Sines (SSA Condition)

When two sides and a non-included angle are known (SSA), there can be zero, one, or two possible triangles. Recognizing this ambiguity is crucial to identify all possible solutions for the unknown angles in the given triangle.
추천 영상:
9:50
Solving SSA Triangles ("Ambiguous" Case)