1/(1×3×5)+1/(3×5×7)+1/(5×7×9)+1/(7×9×11)+1/(9×11×13)
=(1/2)*[1/(1*3)-1/(3*5)]+(1/2)*[1/(3*5)-1/(5*7)]+...+(1/2)*[1/(9*11)-1/(11*13)]
=(1/2)*[1/(1*3)-1/(11*13)]
=(1/2)*(140/429)
=70/429.
1/(1×3×5)+1/(3×5×7)+1/(5×7×9)+1/(7×9×11)+1/(9×11×13)
=(1/2)*[1/(1*3)-1/(3*5)]+(1/2)*[1/(3*5)-1/(5*7)]+...+(1/2)*[1/(9*11)-1/(11*13)]
=(1/2)*[1/(1*3)-1/(11*13)]
=(1/2)*(140/429)
=70/429.