a,=2.(1+11+111+...........+111...11(50))
=2.(9+99+999+......+.999.....9999(50)/9
=2.(10-1+10^2-1+....................+10^50-1)/9
=2.((1+10+10^2+....+10^50−1)−17/9
=2. (10^51-1/9)-18/9
B=1/1.2.3+1/2.3.4+.............+1/97.98.99+1/98.99.100
2B=2/1.2.3+2/2.3.4+.............+2/98.99.100
2B=1/1.2-1/2.3+1/2.3-1/3.4+....................+1/98.99-1/99.100
2B=1/1.2-(1/2.3-1/2.3)-..............................-(1/98.99-1/98.99)-1/99.100
2B=1/1.2-0-0-0-0-........-0-1/99.100=1/1.2-1/99.100
2B=1/2-1/9900=4950/9900-1/9900=4949/9900
B=4949/9900 .1/2=4949/19800
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