y'=x^(-1/2),
设P(x0,y0)处切线与直线垂直,直线2x+y-3=0斜率k1=-2
则切线斜率k2为其负倒数,为1/2,
x0^(-1/2)=1/2,
x0=4,y0=5,
所以曲线在P(4,5)处的切线与直线2x+y-3=0相垂直.