Tính 5^2+11^2+17^2+...+305^2
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1: =1/5-1/4=-1/20
2: =6/8-7/8=-1/8
3: =11/18-14/18=-3/18=-1/6
4: =-28/210+90/210+28/210=3/7
5: =-8/17-34/17=-42/17
1)\(\dfrac{1}{5}+\dfrac{-1}{-4}=\dfrac{4}{20}+\dfrac{5}{20}=\dfrac{9}{20}\)
\(\)2)\(\dfrac{3}{4}+\dfrac{-7}{8}=\dfrac{6}{8}+\dfrac{-7}{8}=\dfrac{-1}{8}\)
3)\(\dfrac{11}{18}+\dfrac{-7}{9}=\dfrac{11}{18}+\dfrac{-14}{18}=\dfrac{-3}{18}=\dfrac{-1}{6}\)
4)\(\dfrac{2}{-15}+\dfrac{3}{7}+\dfrac{-4}{-30}=\dfrac{-2}{15}+\dfrac{2}{15}+\dfrac{3}{7}=0+\dfrac{3}{7}=\dfrac{3}{7}\)
5)\(\dfrac{8}{-17}+\left(-2\right)=\dfrac{-8}{17}+\dfrac{-34}{17}=\dfrac{-42}{17}\)
9/5^2<9/2.8
9/11^2<9/8.14
............
9/305^2<9/302.308
=>B<9/2.8+9/8.14+......+9/302.308
=9/6(1/2-1/8+1/8-1/14+..........+1/302-1/308
=3/2(1/2-1/308)<3/2.1/2=3/4(đpcm)