3.7+3<3.7×3
0.9÷1.5<0.9×1.5
9.99÷0.01>999×0.01
0.21×0.21<0.21÷0.21
5.4÷0.2=5.4×5
1.78×3.3>17.8×0.033
0.8÷1.6<0.8×1.6.
故答案为:<,<,>,<,=,>,<.
3.7+3<3.7×3
0.9÷1.5<0.9×1.5
9.99÷0.01>999×0.01
0.21×0.21<0.21÷0.21
5.4÷0.2=5.4×5
1.78×3.3>17.8×0.033
0.8÷1.6<0.8×1.6.
故答案为:<,<,>,<,=,>,<.