I know that I need to use separations of variables. After solve, how to apply the initial value?
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Solve to find the general solution, then find the value of the constant:
y' = 2x / (1 + 2y)
dy / dx = 2x / (1 + 2y)
(1 + 2y) dy = 2x dx
∫ (1 + 2y) dy = ∫ 2x dx
y + y² = x² + C
When x = 2, y = 0
0 + 0² = 2² + C
0 = 4 + C
C = -4
y + y² = x² - 4
y² + y = x² - 4
4y² + 4y = 4x² - 16
(2y + 1)² - 1 = 4x² - 16
(2y + 1)² = 4x² - 15
2y + 1 = ±√(4x² - 15)
2y = -1 ± √(4x² - 15)
y = [-1 ± √(4x² - 15)] / 2
y' = 2x / (1 + 2y)
dy / dx = 2x / (1 + 2y)
(1 + 2y) dy = 2x dx
∫ (1 + 2y) dy = ∫ 2x dx
y + y² = x² + C
When x = 2, y = 0
0 + 0² = 2² + C
0 = 4 + C
C = -4
y + y² = x² - 4
y² + y = x² - 4
4y² + 4y = 4x² - 16
(2y + 1)² - 1 = 4x² - 16
(2y + 1)² = 4x² - 15
2y + 1 = ±√(4x² - 15)
2y = -1 ± √(4x² - 15)
y = [-1 ± √(4x² - 15)] / 2
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dy/dx = (2x)/(1 + 2y), y(2) = 0
dy/(1 + 2y) = 2x dx
Integrating both sides:
1/2*ln|1 + 2y| = x² + C
1 + 2y = C*e^(2x²)
y = C*e^(2x²) - 1/2
0 = C*e^(8) - 1/2
C = 1/(2e^8)
y = 1/2*e^(2x² - 8) - 1/2
dy/(1 + 2y) = 2x dx
Integrating both sides:
1/2*ln|1 + 2y| = x² + C
1 + 2y = C*e^(2x²)
y = C*e^(2x²) - 1/2
0 = C*e^(8) - 1/2
C = 1/(2e^8)
y = 1/2*e^(2x² - 8) - 1/2