7. …=[(2а-b) (2a+b)]/[6a*3*(2a-b) ]=(2a+b) /18a
8. …=[(7a-b) (7a+b)] /[8a*2*(7a-b)] =(7a+b) /16a
9. …=7a/[(a-5b) (a+5b)] *b*(a+5b) =7ab/(a-5b)
10.…=2a/[(a-6b) (a+6b)] *b*(a-6b) =2ab/(a+6b)
[а/(аb-5b^2)]:[5a^2/(a^2-10ab+25b^2)]=[a/(b(a-5b))] :[5a^2/(a-5b) ^2]=[a/(b(a-5b))] *[(a-5b) ^2/5a^2]=(a-5b) /5ab