8 + 88 + 888 + .............n terms = ____
8081(10n−1)−8n9
S=8+88+888+..... =8[1+11+111....] =89[9+99+999+......] =89[(10−1)+(102−1)+(103−1)+.....] =89[(10+102+103+......)−(1+1+1+......)] =89[10×10n−110−1−n]
=89[10×10n−19−n]
=8081(10n−1)−8n9