Let be a right-angled triangle with as the hypotenuse. Lengths of and AC are and , respectively. The minimum possible time, in minutes, required to reach the hypotenuse from A at a speed of per hour is
Let be a right-angled triangle with as the hypotenuse. Lengths of and AC are and , respectively. The minimum possible time, in minutes, required to reach the hypotenuse from A at a speed of per hour is
Entered answer:
Solution
We need to find the shortest path from point A to the hypotenuse BC.
Since ABC is a right-angled triangle with AB = 15 km and AC = 20 km, we can use the Pythagorean theorem to find BC:
BC² = AB² + AC²
BC² = 15² + 20²
= 225 + 400
= 625
BC = 25 km
Notice that 15, 20, 25 is just the famous 3-4-5 right triangle scaled up by 5!
The question asks for the minimum time to reach the hypotenuse. This means we need the shortest distance from A to line BC.
The shortest distance from any point to a line is always the perpendicular distance (the altitude). If you draw any other line from A to BC, it will be longer than the perpendicular.
For any right triangle, we can find the altitude to the hypotenuse using the area formula.
Area of triangle = × base × height
We can calculate the area in two ways:
Using the two legs: Area = × AB × AC = × 15 × 20 = 150 km²
Using hypotenuse and altitude: Area = × BC × altitude
Since both give the same area:
150 = × 25 × altitude
altitude = 300 ÷ 25 = 12 km
In a 3-4-5 triangle, the altitude to hypotenuse = = 2.4. In our 15-20-25 triangle (scaled by 5), it's 2.4 × 5 = 12 km
Now we use the basic formula: Time = Distance ÷ Speed
Time = 12 km ÷ 30 km/hr = hours = hours
Converting to minutes: × 60 = 24 minutes
Therefore, the minimum possible time is 24 minutes.
When finding the shortest path from a point to a line, always think "perpendicular distance" - it's a concept that appears frequently in geometry problems!