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The points \(A(2, 1)\), \(B(7, 1)\), and \(C(8, 4)\) are shown on a coordinate plane.
a) Find the coordinates of a fourth point \(D\) so that \(ABCD\) is a parallelogram.
b) Find the length of the height \(h\) perpendicular to side \(AB\).
c) Find the area of parallelogram \(ABCD\).
Hints
- What is true about opposite sides of a parallelogram?
- What is the vertical distance from the lower side to the upper side?
- What formula gives the area of a parallelogram?
Solution
1. Segment \(AB\) moves \(5\) units to the right. In a parallelogram, the opposite side must be parallel and equal in length, so move \(5\) units left from \(C(8, 4)\). This gives \(D(3, 4)\).
2. Side \(AB\) lies on \(y=1\), and side \(CD\) lies on \(y=4\). Their perpendicular distance is \(4-1=3\) units, so \(h=3\) units.
3. The base is \(7-2=5\) units. Therefore, \(A=bh=5\times3=15\) square units.
Answer
a) \(D(3, 4)\)
b) \(h=3\) units
c) \(A=15\) square units
