√3×√1/3
=√(3×1/3)
=√1
=1.
√3/√12
=√(3/12)
=√(1/4)
=1/2.
（√2+√5）^2
=(√2)^2 +2√2×√5 +(√5)^2
=2 +2√10 +5
=7 +2√10
√50×√8-21
=√(50×8) -21
=√400 -21
=20 -21
= -1.