Ta có: 11 + 11 = (1+1) . (1+1) = 2 . 2 = 4
12 + 13 = (1+2) . (1+3) = 3 . 4 = 12
=> 21 + 14 = (2+1) . (1+4) = 3 . 5 = 15
ta co 1+1=2
=>11+11=2*2=4
1+2=3
1+3=4
=>12+13=3*4
2+1=3
1+4=5
=>21+14=3*5=15
Ta có quy luật sau :
11 + 11 = 4
=> 2 x 2 = 4
12 + 13 = 12
=> 3 x 4 = 12
Do đó : 21 x 14 = 3 x 5 = 15
11 + 11 = 4
-> 2 x 2 = 4
12 + 13 = 12
-> 3 x 4 = 12
21 + 14
-> 3 x 5 = 15