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