Solution
Related Formula
For differential equations where terms are functions of a linear expression like (x+y), substitute a new variable t = x+y to enable variable separation.
Core Logic
Given the differential equation:
(dy)/(dx) = 2x(x+y)³ - x(x+y) - 1Let x+y = t 1 + (dy)/(dx) = (dt)/(dx) (dy)/(dx) = (dt)/(dx) - 1.
Substituting these terms back:
(dt)/(dx) - 1 = 2xt³ - xt - 1 (dt)/(dx) = 2xt³ - xt (dt)/(dx) = xt(2t² - 1)Step 1: Separating Variables and Integrating
Separating variables:
(dt)/(t(2t² - 1)) = x dxMultiply numerator and denominator by t:
(t dt)/(t²(2t² - 1)) = x dxLet t² = z 2t dt = dz t dt = (dz)/(2):
∫ (dz)/(2z(2z-1)) = ∫ x dx ∫ (dz)/(z(2z-1)) = ∫ 2x dxUsing partial fractions:
∫ ( (2)/(2z-1) - (1)/(z) ) dz = ∫ 2x dx ln|2z-1| - ln|z| = x² + C ln|(2z-1)/(z)| = x² + CStep 2: Apply the Boundary Condition
Given y(0) = 1 at x = 0, y = 1 t = 0 + 1 = 1 z = t² = 1. Substituting these value constraints into our integral solution:
ln|(2(1)-1)/(1)| = 0² + C ln(1) = C C = 0Thus:
(2z-1)/(z) = ex² 2 - (1)/(z) = ex² (1)/(z) = 2 - ex² z = 12 - ex²Since z = t² = (x+y)², we have:
(x+y)² = 12 - ex²Step 3: Evaluate at the Target Value
We need to find the value of the function at x = 1√(2):
( 1√(2) + y( 1√(2)) )² = 12 - e^( 1√(2))² = 12 - e1/2 = 12 - √(e)Pattern Recognition
Sees: Differential equation format y' = f(x+y). Shortcut: A linear argument (x+y) strongly implies substituting t=x+y. The final expression requested matched the functional template (x+y)² exactly, saving steps from extracting standalone square roots.
Chapter Mix
Class 12 Mathematics: Differential Equations