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[tex] \frac{17}{42} + \frac{18}{42} +\frac{19}{42} +...+ \frac{84}{42} [/tex]

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17/42+18/42+19/42+...+84/42 =
[tex]\frac{17+18+19+...+84}{42}=\\
=\frac{(1+2+...16)+17+18+...+84 \;-\;(1+2+3+...+16)}{42}=\\
=\frac{84*85:2\;-\;16*17:2}{42}=\\
=\frac{3570-136}{42}=\\
=\frac{3434}{42}=\frac{1717}{21}[/tex]

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