Solution for Question 3(b)
Finding the asymptotes of the cubic curve:
\[ 2x^3 – 5x^2y + 4xy^2 – y^3 + 6x^2 – 7xy + y^2 – x + 5y – 3 = 0 \]
Step 1: Group Homogeneous Terms
- Degree 3 (\(\phi_3\)): \(2x^3 – 5x^2y + 4xy^2 – y^3\)
- Degree 2 (\(\phi_2\)): \(6x^2 – 7xy + y^2\)
- Degree 1 (\(\phi_1\)): \(-x + 5y\)
Step 2: Find the Slopes (\(m\))
Put \(x=1, y=m\) in \(\phi_3\):
\[ \phi_3(m) = 2 – 5m + 4m^2 – m^3 = 0 \]
Solving for \(m\):
\[ -(m-1)^2(m-2) = 0 \]
The roots are \(m = 1, 1, 2\).
Step 3: For Non-Repeated Root (\(m = 2\))
Using \(c = -\frac{\phi_2(m)}{\phi’_3(m)}\):
- \(\phi_2(2) = 6 – 7(2) + (2)^2 = -4\)
- \(\phi’_3(m) = -5 + 8m – 3m^2 \implies \phi’_3(2) = -1\)
Step 4: For Repeated Roots (\(m = 1, 1\))
Using the quadratic formula for \(c\):
\[ \frac{c^2}{2!}\phi”_3(1) + \frac{c}{1!}\phi’_2(1) + \phi_1(1) = 0 \]
Values at \(m=1\):
Asymptotes 2 & 3: \(x – y + 1 = 0\) and \(x – y + 4 = 0\)
- \(\phi”_3(1) = 8 – 6(1) = 2\)
- \(\phi’_2(1) = -7 + 2(1) = -5\)
- \(\phi_1(1) = -1 + 5(1) = 4\)
Asymptotes 2 & 3: \(x – y + 1 = 0\) and \(x – y + 4 = 0\)
Final Asymptotes:
1. \(2x – y – 4 = 0\)
2. \(x – y + 1 = 0\)
3. \(x – y + 4 = 0\)
1. \(2x – y – 4 = 0\)
2. \(x – y + 1 = 0\)
3. \(x – y + 4 = 0\)
