y'=(x^(1/2))'=(1/2)x^(-1/2)
y-y0=y'(x-x0)
过(0,2)
y-√x0=(1/(2√x0))(x-x0)
2-√x0=(1/(2√x0))(0-x0)
x0=16
y-4=(1/8)(x-16)