Math Problem Statement

如右圖,已知 G 點為△ABC 的重心,D 點為 BC 的中點,以 G 點為圓心,GD 為半徑畫一圓,且作 A 點到此圓的兩切線段 AE 與 AF,其中 E、F 為切點。若 ∠B = 40°,∠C = 45°,則 ∠1 + ∠2 = ?

Solution

The problem in the image presents a geometric figure of triangle ABC\triangle ABC, with several specific points and conditions provided. Here's a breakdown of the key points:

  1. GG is the centroid of triangle ABCABC, and DD is the midpoint of side BCBC.
  2. A circle is drawn with GG as the center and GDGD as the radius.
  3. Two tangents from point AA touch the circle at points EE and FF, creating line segments AEAE and AFAF.
  4. The angles at vertex BB and vertex CC are given as B=40\angle B = 40^\circ and C=45\angle C = 45^\circ, respectively.
  5. We are asked to find the sum 1+2\angle 1 + \angle 2, where 1\angle 1 and 2\angle 2 are the angles formed by the tangents from AA to the circle (as indicated in the diagram).

Solution Approach:

  • First, note that GG, being the centroid, divides the medians of the triangle in a 2:1 ratio, and the circle drawn with GG as the center implies some symmetry.
  • Since the two tangents from point AA meet the circle at points EE and FF, the property of tangents to a circle (that they are equal in length and the angles subtended by them are equal) plays a role.
  • The angles 1\angle 1 and 2\angle 2 are equal because of the symmetry of the tangents AEAE and AFAF.

Using the angle sum property in triangle ABCABC: A+B+C=180\angle A + \angle B + \angle C = 180^\circ A=180(40+45)=95\angle A = 180^\circ - (40^\circ + 45^\circ) = 95^\circ Thus, A=95\angle A = 95^\circ.

Since A=1+2\angle A = \angle 1 + \angle 2 (due to the symmetry of tangents from point AA to the circle), the sum 1+2\angle 1 + \angle 2 is: 95\boxed{95^\circ}

Would you like more details or have any questions about the steps? Here are some related questions:

  1. How do we use the properties of tangents to solve such problems?
  2. What is the centroid, and how does it divide the medians of a triangle?
  3. How does the angle sum property of triangles help in geometry problems?
  4. How can symmetry in geometric figures simplify solving for unknown angles?
  5. What is the significance of the incenter and circumcenter in triangle geometry?

Tip: Always look for symmetrical properties when working with tangents and circles in geometry.

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Math Problem Analysis

Mathematical Concepts

Geometry
Circle Properties
Tangents
Centroid of a Triangle

Formulas

Angle Sum Property of a Triangle: ∠A + ∠B + ∠C = 180°

Theorems

Properties of Tangents from a Point to a Circle
Angle Sum Property of a Triangle

Suitable Grade Level

Grades 10-12