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Solve ΔABC using the Law of Sines

1. A = 29°, C = 63°, c = 24

2. A = 72°, B= 35°, c = 21

1 Answer

5 votes

Answer:

1) B = 88°, a = 13.1, b = 26.9

2) C = 73°, a = 20.9, b = 12.6

Explanation:

To solve for the remaining sides and angles of the triangle, given two sides and an adjacent angle, use the Law of Sines formula:


\boxed{\begin{minipage}{7.6 cm}\underline{Law of Sines} \\\\$(a)/(\sin A)=(b)/(\sin B)=(c)/(\sin C)$\\\\\\where:\\ \phantom{ww}$\bullet$ $A, B$ and $C$ are the angles. \\ \phantom{ww}$\bullet$ $a, b$ and $c$ are the sides opposite the angles.\\\end{minipage}}

Question 1

Given values:

  • A = 29°
  • C = 63°
  • c = 24

As the interior angles of a triangle sum to 180°:


\implies A+B+C=180^(\circ)


\implies B=180^(\circ)-A-C


\implies B=180^(\circ)-29^(\circ)-63^(\circ)


\implies B=88^(\circ)

Substitute the values of A, B, C and c into the Law of Sines formula and solve for sides a and b:


\implies (a)/(\sin A)=(b)/(\sin B)=(c)/(\sin C)


\implies (a)/(\sin 29^(\circ))=(b)/(\sin 88^(\circ))=(24)/(\sin 63^(\circ))

Solve for a:


\implies (a)/(\sin 29^(\circ))=(24)/(\sin 63^(\circ))


\implies a=(24\sin 29^(\circ))/(\sin 63^(\circ))


\implies a=13.0876493...


\implies a=13.1

Solve for b:


\implies (b)/(\sin 88^(\circ))=(24)/(\sin 63^(\circ))


\implies b=(24\sin 88^(\circ))/(\sin 63^(\circ))


\implies b=26.9194211...


\implies b=26.9


\hrulefill

Question 2

Given values:

  • A = 72°
  • B = 35°
  • c = 21

As the interior angles of a triangle sum to 180°:


\implies A+B+C=180^(\circ)


\implies C=180^(\circ)-A-B


\implies C=180^(\circ)-72^(\circ)-35^(\circ)


\implies C=73^(\circ)

Substitute the values of A, B, C and c into the Law of Sines formula and solve for sides a and b:


\implies (a)/(\sin A)=(b)/(\sin B)=(c)/(\sin C)


\implies (a)/(\sin 72^(\circ))=(b)/(\sin 35^(\circ))=(21)/(\sin 73^(\circ))

Solve for a:


\implies (a)/(\sin 72^(\circ))=(21)/(\sin 73^(\circ))


\implies a=(21\sin 72^(\circ))/(\sin 73^(\circ))


\implies a=20.8847511...


\implies a=20.9

Solve for b:


\implies (b)/(\sin 35^(\circ))=(21)/(\sin 73^(\circ))


\implies b=(21\sin 35^(\circ))/(\sin 73^(\circ))


\implies b=12.5954671...


\implies b=12.6

User Malik Khalil Ahmad
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