Question

Va rog ajutați mă repede ​

Answer 1

Răspuns:

Explicație pas cu pas:

a)

1/1×4=1/3-1/3×4

1/4×7=1/3×4-1/3×7

1/7×1-=1/3×7-1/3×10

..................................

1/(3n-2)(3n+1)=1/3(3n-1)-1/3(3n+1)

S=1/3-1/3(3n+1)=(3n+1-1)/3(3n+1)=3n/3(3n+1)=n/(3n+1)

b.

1/1×5=1/4-1/4×5

1/5×9=1/4×5-1/4×9

1/9×13=1/4×9-1/4×13

......................................

1/(4n-3)(4n+1)=1/4×(4n-3)-1/4(4n+1)=1/4-1/4(4n+1)=(4n+1-1)/4(4n+1)=4n/4(4n+1)=

S=n/(4n+1)