The sum of first 20 terms of the sequence 0.5, 0.55, 0.555,...., is ___.
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Solution
Let S =0.5+0.55+0.555+...upto 20 terms=510+55100+555103+...+upto 20 terms =5[110+11100+111103+...+upto 20 terms]
Multiplying and dividing the RHS by 9, we get S=59[((1−110)+(1−(110)2)+(1−(110)3))+.....upto 20 terms]=59[20−110(1−(110)20)1−110]=59[1799+19(110)20]=581[179+(10)−20]