For the differential equation , with initial condition , find . If , for co-prime integers and , find .
Notation: denotes the imaginary unit .
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The given differential equation is the first-order ODE of the form d x d y + p ( x ) y = g ( x ) . We can use a multiplier μ ( x ) = e ∫ p ( x ) d x = e ∫ i d x = e i x . Then we have:
e i x d x d y + i e i x y d x d ( e i x y ) ⟹ e i x y ⟹ y ( x ) y ( 0 ) ⟹ y ( x ) y ( π ) = 2 e i x sin x = 2 e i x ( 2 i e i x − e − i x ) = i ( 1 − e 2 i x ) = ∫ i ( 1 − e 2 i x ) d x = i x − 2 e 2 i x + C = i x e − i x − 2 e i x + C e − i x = 0 − 2 1 + C = 2 3 = i x e − i x − 2 e i x + 2 e − i x = i π e − π i − 2 e π i + 2 e − π i = − π i + 2 1 − 2 = − 2 3 − π i By Euler’s formula: e i θ = cos θ + i sin θ where C is the constant of integration, ⟹ C = 2 By Euler’s formula: e ± π i = − 1
Therefore, a b c = 6 .