3.3x6.6=3.3x6十3.3X0.6=19.8十1.98=21.789.9x7.8=(10一0.1)x7.8=10X7.8一0.1x7.8=78一0.78=77.22
3.3×6.6+9.9×7.8=3.3(3×2.2)+9.9×7.8=9.9×2.2+9.9×7.8=9.9(2.2+7.8)=9.9×10=99