- Difficulty
- Introductory
- Suggested time
- 3 min
- Marks
- 2
Slope fields and Euler’s method practice question 4
The slope field of is shown, with the point marked. On which interval is the solution through increasing?
Along the solution, has the sign of . Starting at , can the solution ever reach the line , where every segment is flat?
The horizontal line is itself a solution, so the solution through stays above it and . Which factor then decides the sign of ?
Theory UBC Math 100 §12.2Key results
Worked solution
Strategy
Find the sign of the slope along the solution. One factor depends only on x; the other stays positive, because the solution never reaches y = −1.
Working
The horizontal line y = −1 is itself a solution, so the solution through (0, 1) stays above it and y + 1 > 0.
With y + 1 > 0, only the factor x − 1 decides.
The solution falls until x = 1, where its segment is flat, and rises after it.
Answer
The answer is x greater than 1.
Check
At (0, 1) the slope is −2, while at (2, 1), where this solution returns by symmetry about x = 1, it is +2: falling, then rising.
Where this goes wrong
Deciding from the slope at the starting point alone. At (0, 1) the slope is −2, but the field changes from point to point: following the segments to the right, they level off at x = 1 and then point upward.
Leans on
The first derivative test, from Curve sketching
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