a²+2b²+3c²+3ab+4ac+5bc=(a²+b²+c²+2ab+2ac+2bc)+b²+2c²+3bc+ab+2ac=（a+b+c)²+（b+2c)(b+c)+a(b+2c)=(a+b+c)²+（a+b+c)(b+2c)=(a+b+c)(a+2b+3c)