casio khó nè

E

eye_smile

AD $\dfrac{3}{a.(a+1).(a+2).(a+3)}$ = $\dfrac{1}{a(a+1).(a+2)}$ - $\dfrac{1}{(a+3)(a+1).(a+2)}$
KQ: $\dfrac{1}{3}$ ( $\dfrac{1}{6}$ - $\dfrac{1}{2012.2013.2014}$ )
 
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