rule
f(0)=f(ζ)+f'(ζ)(0-ζ)+f''(ζ)/2*ζ^2=- 1+f''(ζ)/2*ζ^2;
f( 1)=f(ζ)+f'(ζ)( 1-ζ)+f''(ζ)/2*( 1-ζ)^2=- 1+f''(ζ)/2*( 1-ζ)^2
Two formulas, one can get zeta when [0, 1/2], the other can get zeta when [1/2, 1], and the other can get zeta when [f'' (zeta) ≥