由已知a = (1,0),b = (1,1),c = (-1,0)代入c = xa + yb
得:(-1,0) = x(1,0) + y(1,1)
(-1,0) = (x,0) + (y,y)
(-1,0) = (x + y,y)
有-1 = x + y,0 = y
所以x = -1,y = 0