y = lnx
y' = 1/x
y'|(x=2) = 1/2
当x = 2,y = ln2
切线方程为：y - ln2 = (1/2)(x - 2)
即x - 2y + 2ln2 - 2 = 0
lim(x-->0) sin(5x)/(2x)
= lim(x-->0) [sin(5x)/(5x)] · 5/2
= 1 · 5/2
= 5/2