7+77+777+7777+....+7777.......7(n-digit)=781(10n+1-9n-10)
plzz...help to solve it..i m not getting the L.H.S. equation...
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3 Answers
1357taking 7 common we get
7*(1+11+111+1111.......)
divide and multiply by 9 we get
(7/9)*(9+99+999+9999......)
=(7/9)(10-1+100-1+1000-1+1000-1......)
Proceed from here
269sir i started this way only..but at the end i got the same formula of R.H.S. ....can i generalize the formula in the L.H.S. ...
1357(7/9)(10-1+100-1+1000-1+1000-1......)
=(7/9)(10+100+1000+10000......-n)
=(7/9)(10+102+103...till n terms. - n)
=(7/9)[10*(10n-1)/(10-1) - n]......summation of GP
solve it from here
- Astha Gupta thanq sir...Upvote·0· Reply ·2014-01-18 05:14:22
- Astha Gupta sir, one question.in the second step..how did u get 10,100....???...i didn't understand ...plz xplain..