因式分解x^2+3xy+2y^2+4x+5y+3 由于x^2+3xy+2y^2=(x+2y)(X+y）,而4x+5y拆为x+2y和3（x+y）,而3=1乘3由十字相乘法,可得原式=（x+2y+3）（x+y+1）但我看不懂【由十字相乘法,可得原式=（x+2y+3）（x+y+1）】,

x^2+3xy+2y^2+4x+5y+3
= x^2+(2+1)xy+2y^2+4x+5y+3
=x^2+2xy+xy+2y^24x+5y+3
=x(x+2y)+y(x+2y)+4x+5y+3
= (x+y)(x+2y)+4x+5y+3
=(x+y)(x+2y)+x+2y+3x+3y+3
=(x+y)(x+2y)+(x+2y)+3(x+y+1)
=(x+y+1)(x+2y)+3(x+y+1)
=(x+y+1)(x+2y+3)

x^2+3xy+2y^2
1 2
1 1
对角乘后相加

[（x+2y）+3]* [(x+y)+1]你把小括号先看成整体.这样不就好理解了