Verified answer

12.  ...=(16sqrt8)/((sqrt8)*(sqrt8)) = (16sqrt8)/8 = 2sqrt8

14. ...= -(5((sqrt1,4)+(sqrt1,6)))/((sqrt1,4)-(sqrt1,6))*((sqrt1,4)+(sqrt1,6))=

= -5*(sqrt1,4  +sqrt1,6)/(1,4-1,6) = -5(sqrt1,4  +sqrt1,6)/(-0,2)=25(sqrt1,4  +sqrt1,6)

15.  ...=sqrt(5+sqrt(15+5*0,2))=sqrt(5+sqrt16)=sqrt(5+4)=3

Решение на листке----------