（1）74.05+24.65-6.83
=74+24+0.05+0.65-6.7-0.13
=98+0.7-6.7-0.13
=92-0.13
=91.87
（2）12-8.999-0.037
=12-9-0.037+0.001
=3-0.036
=2.964
（3）4.08-（6.32-0.86-3.72）
=4.08-6.32+0.86+3.72
=4.08+0.86+3.72-6.32
=4.94-2.6
=2.34