设十位数(或以上)为X, 个位数为Y:
(10*X + Y)*(10*X + (10-Y))
= 100*X*X + 100*X - 10*X*Y + 10*X*Y + Y*(10-Y)
= X*(X+1)*100+ Y*(10-Y)
所以, 百位=X*(X+1) 十位和个位= Y*(10-Y)
so :
77×73 =7*8*100 + 3*7=5621
85×85 =8*9*100 + 5*5=7225
94×96 =9*10*100 + 4*6 = 9024
101×109 =10*11*100 + 1*9 = 11009
195×195 =19*20*100 + 5*5=38025
204×206 =20*21*100 + 4*6 = 42024 |