wenn es unbedingt Induktion sein muss, gehtnat. auch:
∑k+1n=2 2 / (n2-1) = 2/ ( (k+1)^2 - 1 ) + ∑kn=2 2 / (n2-1)
= 2/ ( (k+1)^2 - 1 ) + 1 + 1/2 - 1/k - 1/(k+1)
= 1 + 1/2 - 1/(k+1) - 1/k + 2/ ( (k+1)^2 - 1 )
= 1 + 1/2 - 1/(k+1) - 1/k + 2/ ( (k^2 +2k )
= 1 + 1/2 - 1/(k+1) - (k+2) /k(k+2) + 2/ ( (k^2 +2k )
= 1 + 1/2 - 1/(k+1) + 2/ ( (k^2 +2k ) - (k+2) /k(k+2)
= 1 + 1/2 - 1/(k+1) + (2 - k - 2) / (k^2 +2k )
= 1 + 1/2 - 1/(k+1) - k / (k^2 +2k )
= 1 + 1/2 - 1/(k+1) - 1 / (k +2 ) passt also !