(2√7-5√2)(√7-√8)=2√7*√7-2√7*√8-5√2√7+5√2*√8=2*7-2√56-5√14+5*4=
=14-√14(2√4+5)+20=34-√14(4+5)=34-9√14
√(0.81a^2)+√(1/4b^2)=0.9a+0.5b
(9-x)(√x-3)=-(x-9)/(√x-3)=-(√x-3)(√x+3)/(√x-3)=-(√x+3)=-√x-3
1/(5√2-3)-1/(5√2+3)=(5√2+3-(5√2-3))/((5√2-3)(5√2+3))=(3+3)((5√2)^2-3^2)=6/(25*2-9=6/41