4) a) [tex] x^{2} [/tex]+6xy+9[tex] y^{2} [/tex]+[tex] x^{2} [/tex]-9[tex] y^{2} [/tex]=
=2[tex] x^{2} + 6xy[/tex]
b) [tex]16 x^{2} -24x+9+ 5x^{2}-15x=21 x^{2} -39x+9 [/tex]
c) [tex]1+2x+ x^{2} +4+4x+ x^{2} +9+6x+ x^{2} =3 x^{2} +12x+14[/tex]
d)[tex] x^{4} +2 x^{2}y+ y^{2} - x^{4} +2 x^{2} y- y^{2} =4 x^{2} y[/tex]
