1) (6a^2 + 5b^2)(2a^2-4b^2)2)(-7m^2-8n^2)(-m^2+3n^2)3)(4n^2-1)(n^2+5)4)(8x^2-3xy)(3x^3-xy...

1) $(6a^2+5b^2)(2a^2-4b^2)=12a^4-24a^2b^2+10a^2b^2-20b^4=12a^2-14a^2b^2-20b^2$

2) $-(8n^2+7m^2)(3n^2-m^2)=-(24n^4-8n^2m^2+21n^2m^2-7m^4)=7m^2-13n^2m^2-24n^4$

3) $(4n^2-1)(n^2+5)=4n^4+20n^2-n^2-5=4n^4+19n^2-5$

4) $(8x^2-3xy)(3x^3-xy)=24x^5-8x^3y-9x^4y+3x^2y^2$ $(5ab^3-4b^3)(3ab^3-4a^2)=15a^2b^6-20a^3b^3-12ab^6+16a^2b^3$