3²-1²=4*2=4*[(3+1)/2] 4²-2²=4*3=4*[(4+2)/2] 5²-3²=4*4=4*[(5+3)/2] 所以(n+2)²-n²=4*[(n+2+n)/2]=4*(n+1)(n+2)²-n²=(n+2+n)(n+2-n)=(2n+2)*2=4(n+1)正确
(n+2)²-n² =(n+2+n)(n+2-n) =(n+2）²-n ²=4(n+1) =4n+4
