下图显示了一个顶点为 、、 和 的四边形 。 位于 x 轴上,是边 的中点。

- (a)
解释为什么 是一个梯形。
- (b)
求 的坐标。
- (c)
点 N 在边 BC 上。已知四边形 MNCD 的面积为 平方单位,求 N 的坐标。
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完整解法
9(a)
1. Calculate the gradient of side AB.
Gradient of
2. Calculate the gradient of side DC.
Gradient of
3. Conclude that ABCD is a trapezium based on parallel sides.
Since the gradient of equals the gradient of , the lines and are parallel. A quadrilateral with exactly one pair of parallel sides is a trapezium.
Gradient of and Gradient of . Since the gradients are equal, is parallel to . A quadrilateral with one pair of parallel opposite sides is a trapezium.
9(b)
1. Apply the midpoint formula to points $A$ and $D$
Coordinates of
2. Evaluate the coordinates
9(c)
1. Find the coordinates of $M$ (from part b).
is the midpoint of and .
2. Find the equation of the line $BC$.
Gradient of
Equation of :
3. Express the coordinates of $N$ and set up the area equation for quadrilateral $MNCD$.
Let be .
Using the shoelace formula for the area of with vertices , , , and :
4. Simplify the area equation.
5. Solve for $x$ and find the corresponding $y$-coordinate.
Since lies on the line segment , its -coordinate must be between and . Thus, .
Substitute into the equation of :
The coordinates of are .
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