>201
　A = ([2][1])・([1,1])
　B = ([1][0])・([1,1])

(1)
　X([2][1]) = ([1][0]),
　X =([x,1-2x][z,-2z]).
(2)
　X = ([x,y][z,w]) とおくと
　XAX = X([2][1])・([1,1])X = ([2x+y,2z+w])・([x+z,y+w]).
　∴ (2z+w) = 0,　(2x+y)(x+z) = (2x+y)(y+w) = 1　∴ (x+z) = (y-2z)
　∴ X =([x, ±(√3)-2x][±(1/√3)-x, 干(2/√3)+2x]).
(3)
　BA = ([1][0])・([1,1])([2][1])・([1,1]) = ([1][0])･3･([1,1]) = 3B,　B^n = B より
　3BX = B.
　∴ ([1][0])・3([1,1])X = ([1][0])・([1,1]).
　∴ 3([1,1])X = ([1,1])
　∴ X =([x,y][(1/3)-x,(1/3)-y])