The problem requires us to find the lengths of the sides AB, BC, and AC, and the measures of the angles $\angle ABC$, $\angle BAC$, and $\angle ACB$ for the triangle formed by the points A(3, 4), B(1, 1), and C(5, 1).

GeometryTriangleDistance FormulaLaw of CosinesCoordinate GeometryAngle CalculationSide Length Calculation
2025/3/12

1. Problem Description

The problem requires us to find the lengths of the sides AB, BC, and AC, and the measures of the angles ABC\angle ABC, BAC\angle BAC, and ACB\angle ACB for the triangle formed by the points A(3, 4), B(1, 1), and C(5, 1).

2. Solution Steps

First, we calculate the lengths of the sides using the distance formula.
The distance formula between two points (x1,y1)(x_1, y_1) and (x2,y2)(x_2, y_2) is:
d=(x2x1)2+(y2y1)2d = \sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2}
AB=(13)2+(14)2=(2)2+(3)2=4+9=133.61AB = \sqrt{(1-3)^2 + (1-4)^2} = \sqrt{(-2)^2 + (-3)^2} = \sqrt{4 + 9} = \sqrt{13} \approx 3.61
BC=(51)2+(11)2=(4)2+(0)2=16=4BC = \sqrt{(5-1)^2 + (1-1)^2} = \sqrt{(4)^2 + (0)^2} = \sqrt{16} = 4
AC=(53)2+(14)2=(2)2+(3)2=4+9=133.61AC = \sqrt{(5-3)^2 + (1-4)^2} = \sqrt{(2)^2 + (-3)^2} = \sqrt{4 + 9} = \sqrt{13} \approx 3.61
Next, we find the angles using the Law of Cosines.
The Law of Cosines is:
c2=a2+b22abcos(C)c^2 = a^2 + b^2 - 2ab \cos(C), where a, b, and c are the side lengths, and C is the angle opposite side c.
To find ABC\angle ABC, we use the formula with AC2=AB2+BC22(AB)(BC)cos(ABC)AC^2 = AB^2 + BC^2 - 2(AB)(BC) \cos(\angle ABC):
13=13+162(13)(4)cos(ABC)13 = 13 + 16 - 2(\sqrt{13})(4) \cos(\angle ABC)
0=16813cos(ABC)0 = 16 - 8\sqrt{13} \cos(\angle ABC)
813cos(ABC)=168\sqrt{13} \cos(\angle ABC) = 16
cos(ABC)=16813=213\cos(\angle ABC) = \frac{16}{8\sqrt{13}} = \frac{2}{\sqrt{13}}
ABC=cos1(213)56.31\angle ABC = \cos^{-1}(\frac{2}{\sqrt{13}}) \approx 56.31^\circ
To find BAC\angle BAC, we use the formula with BC2=AB2+AC22(AB)(AC)cos(BAC)BC^2 = AB^2 + AC^2 - 2(AB)(AC) \cos(\angle BAC):
16=13+132(13)(13)cos(BAC)16 = 13 + 13 - 2(\sqrt{13})(\sqrt{13}) \cos(\angle BAC)
16=262(13)cos(BAC)16 = 26 - 2(13) \cos(\angle BAC)
16=2626cos(BAC)16 = 26 - 26 \cos(\angle BAC)
26cos(BAC)=1026 \cos(\angle BAC) = 10
cos(BAC)=1026=513\cos(\angle BAC) = \frac{10}{26} = \frac{5}{13}
BAC=cos1(513)67.38\angle BAC = \cos^{-1}(\frac{5}{13}) \approx 67.38^\circ
To find ACB\angle ACB, we can use the fact that the sum of angles in a triangle is 180180^\circ:
ACB=180ABCBAC=18056.3167.3856.31\angle ACB = 180^\circ - \angle ABC - \angle BAC = 180^\circ - 56.31^\circ - 67.38^\circ \approx 56.31^\circ

3. Final Answer

AB = 13\sqrt{13} ≈ 3.61
BC = 4
AC = 13\sqrt{13} ≈ 3.61
m\angleABC ≈ 56.31^\circ
m\angleBAC ≈ 67.38^\circ
m\angleACB ≈ 56.31^\circ

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