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Calculati urmatoarele sume :


a) S3 = 1/(1*2) + 2/(2*4) + 3/(4*7)+ 4/(7*11) + 5/(11*16)
b) S4 = 1/2 + 1/6 + 1/12 + ... + 1/(n²+n) , n ∈N*


Răspuns :

S3=(1/1 - 1/2)+(1/2 - 1/4)+(1/4-1/7)+(1/7-1/11)+(1/11-1/16)=1-1/16=15/16
S4=(1/1 - 1/2)+(1/2 - 1/3)+(1/3 - 1/4)+(1/4 - 1/5)+.........+(1/n - 1/(n+1)
S4=1-1/(n+1)=n/(n+1)