原式=cosx+sinx
=根号2*sin（x+π/4）
f(x0)=4根号2/5,即
sin（x0+π/4）=4/5
∵x∈（0,π/4）,
∴（x0+π/4）∈（π/4,π/2）,
sin（x0+π/4）=4/5
cos（x0+π/4）=3/5
f(x0+π/6)
=根号2*sin（x0+π/4+π/6)
=根号2*[（4/5*0.5根号3）+3/5*0.5]
=(4*根号6+3*根号2)/10
≈1.404