We look for a pair of terms that multiply to 15y^2 and add properly to 8y and we discover 3y and 5y will work:
X^2+8xy+15y^2=
X^2+x(3y+5y)+15y^2=
X^2+3xy +5xy+15y^2=
(X^2+3xy)+(5xy+15y^2)=
x(x+3y)+5y(x+3y)=
(x+5y)(x+3y)
:)
X^2+8xy+15y^2=
X^2+x(3y+5y)+15y^2=
X^2+3xy +5xy+15y^2=
(X^2+3xy)+(5xy+15y^2)=
x(x+3y)+5y(x+3y)=
(x+5y)(x+3y)
:)
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