(2+1)(2²+1)(2⁴+1)……(2ⁿ+1)
=(2-1)(2+1)(2²+1)(2⁴+1)……(2ⁿ+1)
=(2²-1)(2²+1)(2⁴+1)……(2ⁿ+1)
=(2⁴-1)(2⁴+1)……(2ⁿ+1)
=2²ⁿ -1