f=@(x)(x-6.81)*(x-6.91)*(x-7.01)/((6.71-6.81)*(6.71-6.91)*(6.71-7.01))*375.89+(x-6.71)*(x-6.91)*(x-7.01)/((6.81-6.71)*(6.81-6.91)*(6.81-7.01))*391.65+(x-6.71)*(x-6.81)*(x-7.01)/((6.91-6.71)*(6.91-6.81)*(6.91-7.01))*407.86+(x-6.71)*(x-6.81)*(x-6.91)/((7.01-6.71)*(7.01-6.81)*(7.01-6.91))*424.5;
f(6.71)
f(6.81)
f(6.91)
f(7.01)