Numerical equation solving with FindRoot and BandpassFilter
FindRoot expects a function as first argument.
Try
tf = FunctionInterpolation[Testf[x], {x, .1, .5}]
Plot[tf[x], {x, .1, .5}]
FindRoot[tf[x] == 0, {x, 0.3, 0.4}]
(*{x -> 0.335848}*)
FindRoot expects a function as first argument.
Try
tf = FunctionInterpolation[Testf[x], {x, .1, .5}]
Plot[tf[x], {x, .1, .5}]
FindRoot[tf[x] == 0, {x, 0.3, 0.4}]
(*{x -> 0.335848}*)