2t = [ 1+ 根号(1+4t²)] / 2
4t = 1+ 根号(1+4t²)
4t -1 = 根号(1+4t²) l两边平方得
16t² - 8t + 1 = 1+4t²
12t² - 8t = 0
4t(3t - 2) = 0
∴ t =0 ,t= 2/3