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x in (-oo:+oo)
a = (1/2)*h*(x+y) // - (1/2)*h*(x+y)
a-((1/2)*h*(x+y)) = 0
(-1/2)*h*(x+y)+a = 0
a-1/2*h*(x+y) = 0
a-1/2*h*(x+y) = 0
a-1/2*h*x-1/2*h*y = 0
a-1/2*h*x-1/2*h*y = 0
a-1/2*h*x-1/2*h*y = 0 // - a-1/2*h*y
-1/2*h*x = -(a-1/2*h*y) // : -1/2*h
x = (-(a-1/2*h*y))/(-1/2*h)
x = 2*h^-1*(a-1/2*h*y)
x = 2*h^-1*(a-1/2*h*y)
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