ex=1+x+x22!+x33!+......
Hence
e=1+1+12!+13!+14!..... ...(i)
e−1=1−1+12!−13!+14!.... ...(ii)
Adding i and ii, we get
e+e−1=2[1+12!+14!+..]
And by subtracting ii from i, we get
e−e−1=2[1+13!+15!+...]
Taking the ratio, we get
e+e−1e−e−1=1+12!+14!+..1+13!+15!+..
e2+1e2−1=1+12!+14!+..1+13!+15!+..