99×38+38＝（99+1）×38＝100×38＝3800
99x38+38=(99+1）x38=100x38=3800
(100-1）×38+38=3800-38+38=3800
(99+1)x38=3800
99ⅹ38+38=(99+1)x38=100ⅹ38=3800
