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If the first four terms of an arithmetic sequence are `a,2a,b` and `(a-6-b)` for some numbers a and b, find the sum of the first 100 terms of the sequence. |
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Answer» First four terms of an AP are a, 2a ,b and (a-6-b) So ` " " 2a - a=a-6-b-b` ` rArr " " a= a-6-2b ` `rArr " " - 2b =6rArr b=-3 ` ` and " " 2a-a = b -2a ` ` rArr " " b= 3a rArr a=-1 ` ` therefore ` First terms ` a=-1 and d=a-1 ` ` S_(100) =(100)/( 2) [2a +(100-1)d]` ` " "= 50[-2+99(-1) ]` ` " " 50 (-2-99)= 50)(-101)=- 5050`. |
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