Which of the following best represents the direction field for the differential equation
$$\frac{dy}{dx} = -\frac{x}{4y}$$?
A - HSC - SSCE Mathematics Extension 1 - Question 7 - 2020 - Paper 1
Question 7
Which of the following best represents the direction field for the differential equation
$$\frac{dy}{dx} = -\frac{x}{4y}$$?
A.
B.
C.
D.
Worked Solution & Example Answer:Which of the following best represents the direction field for the differential equation
$$\frac{dy}{dx} = -\frac{x}{4y}$$?
A - HSC - SSCE Mathematics Extension 1 - Question 7 - 2020 - Paper 1
Step 1
Determine the differential equation
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Answer
The equation given is dxdy=−4yx. This suggests that the slope of the field at any point (x, y) depends inversely on y and directly on x.
Step 2
Analyze the implications
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Answer
As y approaches zero, the expression −4yx will approach negative or positive infinity, indicating vertical slopes along the x-axis. This means that at points where y is very small, the direction will change drastically, suggesting a reduction in the slope in the vicinity of the x-axis.
Step 3
Investigate behavior when y is positive or negative
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Answer
For positive values of y, the direction is downward when x is positive and upward when x is negative. For negative values of y, the behavior is reversed: downward when x is negative and upward when x is positive. This establishes a pattern of direction in the field.
Step 4
Select the correct direction field representation
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Answer
Considering all the signs and behaviors from the analysis above, the representation that matches these conditions must be identified. The correct answer is A, which aligns with the deduction of slopes determined earlier.