y =sinh x = [e^x-e^(-x)]/2
2y = e^x-1/e^x
记t=e^x,t＞0那么
2y = t - 1/t
t²-2yt-1 = 0
t = y+√(y²+1)或t = y-√(y²+1)（舍去）
于是e^x = y+√(y²+1)
x = ln[y+√(y²+1)]
因此y = sinh x的反函数为y = ln[x+√(x²+1)].