计算：99×78=（100-1）×78=100×78-1×78=7722
99×78用=(100-1)x78=7800-78=7722
99x78=（100-1）x78=100x78-78x1=7800-78=7722
＝（100-1）x78＝100x78-78＝7800-78＝7722
99×78=（100-1）×78=100×78-1×78=7800-78=7722
