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I confirm this doesn't work but you can substitute by

var('x, y, λ')
f(x, y) = x
c(x, y) = 1 - x^2 - y^2
Λ(x,y,λ) = x + λ*c(x,y)
dΛx = diff(Λ(x,y,x),x)
dΛy = diff(Λ(x,y,λ),y)
dΛλ = diff(Λ(x,y,λ),λ)
solve([dΛx==0,dΛy==0,dΛλ==0],(x, y, λ))