(1/x²+2/x³+1/x^4) ÷(1 +1/x)³=(x²+2x+1)/x^4 ÷[(x+1)/x]³=
= (x+1)²×x³ / (x^4 ×(x+1)³)=1/(x(x+1))=1/(x²+x)
Am egalat aceea expresie pin la sfirsit. Inlocuim {1,2, ..., 11}
S=1/2 +1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56 + 1/72 + 1/90 + 1/110 +
+1/132
Mai departe numai de adus la numitor comun ( mult timp).
