f(x)=4^x/(4^x +2)
f(1-x)=4^(1-x)/[4^(1-x) +2]
=4/(4+2×4^x) /这一步是分子分母同乘以4^x
=2/(4^x +2)
f(x) +f(1-x)=4^x/(4^x +2) +2/(4^x +2)=(4^x +2)/(4^x +2)=1
f(1/7)+f(2/7)+f(3/7)+f(4/7)+f(5/7)+f(6/7)
=[f(1/7)+f(6/7)]+[f(2/7)+f(5/7)]+[f(3/7)+f(4/7)]
=1+1+1
=3