（a+b）^4
=（（a+b）²）²
=（a²+2ab+b²）²
=（（a²+2ab）+b²）²
=（a²+2ab）²+2（a²+2ab）b²+b^4
=a^4+4a³b+4a²b²+2a²b²+4ab³+b^4
=a^4+4a³b+6a²b²+4ab³+b^4