If ft, fr and Lc have been defined previously, the second sentence is clear, unambiguous, conventional and correct.
In mathematical and physical discourse, a function of a variable may be represented as a symbol. For example, we have your ft , or f(t) as it might often be represented. The value f of the function depends on the value of the determining variable t. The symbol ft represents the manner in which the value f of the function depends on the determining variable t. Once defined the symbol is understood as the representation of the functional relationship between the two. It is therefore redundant to use "The" with such defined symbols, whether or not they occur at the start of the sentence.
The alphanumeric expression of the function is irrelevant, as is the number of determining variables. This argument applies to any function. For example F(t), ft, pb, Φx, Gamma(x,y,z),Phiμ.
It is correct usage to refer to the function f(t) in examples such as the following. In each case the function is being defined on its first use.
"Consider the function f(t)=t*t, where f has a quadratic dependence on t."
"The function sin(t) has a regular periodicity in t"