Maths Olympiad Prep

Track / Stage 6 / 341 of 400 #1341 of 1964

Problem 1341

National olympiad, first round
Geometry Difficulty 6.6 Find the answer

Let ABCDABCD be a trapezoid such that AC=8|AC|=8, BD=6|BD|=6, and ADBCAD \parallel BC. Let PP and SS be the midpoints of [AD][AD] and [BC][BC], respectively. If PS=5|PS|=5, find the area of the trapezoid ABCDABCD.

A number or a short expression. Spacing and $ signs are ignored.

Official solution

1. Identify the given information and construct the necessary points:
- Trapezoid ABCDABCD with ADBCAD \parallel BC.
- Diagonals ACAC and BDBD with lengths AC=8|AC| = 8 and BD=6|BD| = 6.
- Midpoints PP and SS of ADAD and BCBC respectively, with PS=5|PS| = 5.

2. **Construct parallelograms CADMCADM and BDMNBDMN:**
- Extend ADAD and BCBC to form parallelograms CADMCADM and BDMNBDMN.
- Since PP and SS are midpoints, PSPS is parallel to both ADAD and BCBC.

3. Analyze the properties of the trapezoid and the midsegment:
- The midsegment PSPS of a trapezoid is parallel to the bases and its length is the average of the lengths of the bases.
- Let AD=aAD = a and BC=bBC = b. Then, PS=a+b2|PS| = \frac{a + b}{2}.

4. **Use the given length of the midsegment to find the relationship between aa and bb:**
- Given PS=5|PS| = 5, we have:
a+b2=5    a+b=10 \frac{a + b}{2} = 5 \implies a + b = 10

5. Determine the height of the trapezoid using the diagonals:
- Since ACBDAC \perp BD, the diagonals intersect at right angles.
- The area of the trapezoid can be found using the formula for the area of a right-angled triangle formed by the diagonals:
Area=12×AC×BD=12×8×6=24 \text{Area} = \frac{1}{2} \times AC \times BD = \frac{1}{2} \times 8 \times 6 = 24

6. Verify the area calculation:
- The area of the trapezoid is indeed given by the product of the diagonals divided by 2, confirming the calculation.

\blacksquare

The final answer is 24 \boxed{ 24 } .

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.