Math Problem Statement

Trong mặt phẳng tọa độ Oxy, cho hình bình hành ABCD có diện tích bằng 2. Biết A(0; 2), B(3; 0) và giao điểm I của hai đường chéo hình bình hành nằm trên đường thẳng y = -x. Tìm hoành độ đỉnh D, biết x_D > -14.

Solution

Let's solve the given problem step by step.

Given Information:

  • Parallelogram ABCDABCD with area 22.
  • Given points: A(0,2)A(0,2) and B(3,0)B(3,0).
  • The intersection of the diagonals II lies on the line y=xy = -x.
  • The xx-coordinate of DD is greater than 14-14.

Step 1: Find the Coordinates of II

Since diagonals of a parallelogram bisect each other, we have:

I=A+C2=B+D2I = \frac{A + C}{2} = \frac{B + D}{2}

Let C(xC,yC)C(x_C, y_C) and D(xD,yD)D(x_D, y_D) be the unknown points.

From the given information, II satisfies y=xy = -x, so let I(a,a)I(a, -a).

I=(0,2)+(xC,yC)2=(3,0)+(xD,yD)2I = \frac{(0,2) + (x_C, y_C)}{2} = \frac{(3,0) + (x_D, y_D)}{2}

Equating components:

(0+xC2,2+yC2)=(a,a)\left( \frac{0 + x_C}{2}, \frac{2 + y_C}{2} \right) = (a, -a)

0+xC2=axC=2a\frac{0 + x_C}{2} = a \Rightarrow x_C = 2a

2+yC2=ayC=2a2\frac{2 + y_C}{2} = -a \Rightarrow y_C = -2a - 2

Similarly, for DD:

(3+xD2,0+yD2)=(a,a)\left( \frac{3 + x_D}{2}, \frac{0 + y_D}{2} \right) = (a, -a)

3+xD2=axD=2a3\frac{3 + x_D}{2} = a \Rightarrow x_D = 2a - 3

0+yD2=ayD=2a\frac{0 + y_D}{2} = -a \Rightarrow y_D = -2a

Step 2: Use the Area Condition

The area of parallelogram ABCDABCD is given by:

S=AB×ADS = \left| \overrightarrow{AB} \times \overrightarrow{AD} \right|

Vectors:

AB=(30,02)=(3,2)\overrightarrow{AB} = (3 - 0, 0 - 2) = (3, -2)

AD=(xD0,yD2)=(2a3,2a2)\overrightarrow{AD} = (x_D - 0, y_D - 2) = (2a - 3, -2a - 2)

Determinant formula:

S=3(2a2)(2)(2a3)=2S = \left| 3(-2a - 2) - (-2)(2a - 3) \right| = 2

6a6+4a6=2\left| -6a - 6 + 4a - 6 \right| = 2

2a12=2\left| -2a - 12 \right| = 2

Solving:

2a12=2or2a12=2-2a - 12 = 2 \quad \text{or} \quad -2a - 12 = -2

2a=14a=7-2a = 14 \Rightarrow a = -7

2a=10a=5-2a = 10 \Rightarrow a = -5

Step 3: Find xDx_D and Choose the Correct One

For a=7a = -7:

x_D = 2(-7) - 3 = -14 - 3 = -17 \quad \text{(Invalid, since x_D > -14)}

For a=5a = -5:

xD=2(5)3=103=13(Valid)x_D = 2(-5) - 3 = -10 - 3 = -13 \quad \text{(Valid)}

Thus, the final answer is:

13\boxed{-13}

Would you like further clarifications? Here are some related questions:

  1. How do you determine the intersection of two diagonals in a parallelogram?
  2. Why does the determinant formula help in finding the area of a parallelogram?
  3. Can this problem be solved using vector cross-product methods?
  4. What happens if the given area was negative instead of positive?
  5. How do you verify the correctness of your calculated coordinates?

Tip: Always check conditions carefully when solving for unknowns, especially when given constraints like xD>14x_D > -14.

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Math Problem Analysis

Mathematical Concepts

Coordinate Geometry
Parallelogram Properties
Linear Equations

Formulas

Area of Parallelogram: |AB × AD|
Midpoint Formula: I = (x1 + x2)/2

Theorems

Diagonal Bisector Theorem of Parallelogram

Suitable Grade Level

Grades 10-12