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Verified answer

8)

(3a^2-5b)(5a^2+b)=15a^4+3a^2b-25a^2b-5b^2=15a^4-22a^2b-5b^2

9)

(y+3)(y^2-2y+5)=y^3-2y^2+5y+3y^2-6y+15=y^3+y^2-y=15

10)

(m+3n)(m^2-6mn-n^2)=m^3-6m^2n-mn^2+3m^2n-18mn^2-3n^3=m^3-3m^2n-19mn^2-3n^3