Observe the following formulas: (x-1) (x + 1) = x2-1 & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; (x-1) (x2 + X + 1) = x3-1 (x-1) (X3 + x2 + X + 1) = x4-1 According to the previous law, we can get the following results: xn + 1 + xn + +x+1）=______ ．

Observe the following formulas: (x-1) (x + 1) = x2-1 & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; (x-1) (x2 + X + 1) = x3-1 (x-1) (X3 + x2 + X + 1) = x4-1 According to the previous law, we can get the following results: xn + 1 + xn + +x+1）=______ ．

According to the meaning of the title: (x-1) (xn + 1 + xn +...) +X + 1) = xn + 2-1