分解因式：
4n^4+1
=(4n^4+4n^2+1)-4n^2
=(2n^2+1)^2-(2n)^2
=(2n^2+2n+1)(2n^2-2n+1)
∵2n^2+2n+1>2n^2-2n+1=2n(n-1)+1>1
∴4n^4+1一定是合数