(5а+6)^2 -81=(5а)²+2*5а*6+6²-81=25а²+60а+36-81=25а²+60а-45
25-(а+7)^2 =25-(а²+2*а*7+7²)=25-(а²+14а+49)=25-а²-14а-49=-а²-14а-24
9m^2-(1+2m)^2=9м²-1-4м-4м²=5м²-4м-1
(5x-3y)^2 -16x^2=25х²-30ху+9у²-16х²=9х²-30ху+9у²
(5c-3d)^2 -9d^2=25с²-30сд+9д²-9д²=25с-30сд
49m^2-(n+8m)^2=49м²-н²-16нм+64м²=113м²-16нм-н²