Answers

2014-03-25T11:05:05+05:30
7+77+777+7777+........+n terms
taking out 7 common
7(1+11+111+1111................)
now multiplying by  \frac{9}{9}
 \frac{7}{9} (9+99+999+9999..............+n terms)
 \frac{7}{9} [(10-1)+(100-1)+(1000-1)..........................]
 \frac{7}{9} [(10+100+1000+10000..........)-(1+1+1+1+1+1.........n)]
 \frac{7}{9} [ \frac{10( 10^{n}-1)}{10-1} -n]
now u can further solve it
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