*1) 9 +9=18.
*2) 99+ 99=198.
*3)999 +999=1998.---------------->(1)
(1)<=> 9(9*2) +9(9*2)=8(9 8)1 1
<=>189 +189=8(17) 2
<=>// +// =78 2 +1
<=>189 +189=378.
*4) 9999 +9999=19998.--------->(1)
(1)<=>9(9*3) +9(9*3)=8(9*2 7)2 +1
(1)<=>279 +279=8(25) 3
(1)<=>// +// =58 3 2
(1)<=>279+ 279 =558.
*5) 99999 99999=199998.------------>(1)
(1)<=>9(9*4)+ 9(9*4)=8(27 6)3 +1
(1)<=>369 +369=8(33) 4
(1)<=>//+ // =38 3 4
(1)<=>369+ 369=738.
*6)<=>999999+ 999999=1999998.-------->(1)
(1)<=>9(9*5)+ 9(9*5) = 8(9*4 5)4 +1
(1)<=>459+ 459=8(41) 5
(1)<=>459+ 459=18 4 5
(1)<=>459 +459=918.
*7) 9999999+ 9999999=19999998.---------->(1)
(1)<=>9(9*6) +9(9*6)=8(9*5 4)5 1
(1)<=>549+ 549=8(49) 6
(1)<=>549+ 549=98 4 6
(1)<=>549+ 549=98 10.
*
99999999 +99999999=199999998.--------->(1)
(1)<=>639+ 639=8(9*6 3)6 +1
(1)<=>639 +639=8(57) 7
(1)<=>639 +639=78 5 7
(1)<=>639 +639=1278.