2 x 9 = (2 - 1)8 = 18
11 x 9 = (11 - 2)9 = 99
13 x 9 = (13 - 2)7 = 117
21 x 9 = (21 - 3)9 = 189
24 x 9 = (24 - 3)6 = 216
91 x 9 = (91 - 10)9 = 819
95 x 9 = (95 - 10)5 = 855
101 x 9 = (101 - 11)9 = 909
106 x 9 = (106 - 11)4 = 954
111 x 9 = (111 - 12)9 = 999
117 x 9 = (117 - 12)3 = 1053
121 x 9 = (121 - 13)9 = 1089
191 X 9 = (191 - 20)9 = 1719
201 X 9 = (201 - 21)9 = 1809
900 X 9 = (900 - 91)|10 = 809|10 = 8100
901 X 9 = (901 - 91)9 = 8109
Rumus Umum :
XY x 9 = [XY - (X + 1)]|(10 - Y)
XYZ x 9 = [XYZ - (XY + 1)]|(10 - Z)
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