1) (√27-√21-√15)/(3-√7-√5)=(√(9*3)-√(7*3)-√(5*3))/3-√7-√5)=
=(3*√3-√7*√3-√5*√3)/(3-√7-√5)=√3*(3-√7-√5)/(3-√7-√5)=√3.
4) 2√3-3∨√(21-12√3)
√(2√3-3)²∨√(21-12√3)
√(12-12√3+9)∨√(21-12√3)
√(21-12√3)≡√(21-12√3).
5)1/(5√2-7)-1/(5√2+7)∨√250
(5√2+7-5√2+7)/((5√2)²-49)∨√250
14/(50-49)∨√250
14/1∨√250
√14²∨√250
√196<√250.