a = 85+90+95+...+160
a = 245+245+245+...+245 (บวกกัน 8 ตัว)
a = 1960
$b \ = \ \frac{1}{2}+\frac{1}{4}+..+\frac{1}{512}$...(1)
$2b \ = \ 1+\frac{1}{2}+\frac{1}{4}+...+\frac{1}{256}$...(2)
$(2)-(1) => b= 1-\frac{1}{512}$
$b \ = \ \frac{511}{512}$
$a+b \ = \ 1960\frac{511}{512}$
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