T= from n=0 to infinity of k^(2n) x ((1x3x5...(2n-1))/(2x4x6x...(2n)))

or

T=[1+(1^2/2^2) k^2 + (1^2x3^2/2^2x4^2) k^4 ...]

notice that all the terms in the series after the first one have coefficients that are at most 1/4. Use this fact to compare this series with a geometric series and show that

(1+k^2/4) < T < (4-3k^2)/(4-4k^2)