2.The definition of an equidistant sequence is: from the 2nd term onwards, the difference between each term and the previous term is the same constant, we call such a sequence an equidistant sequence, and we call this constant the tolerance of the equidistant sequence, which is usually represented by the letter $d$. For example: $a_1=3$, $d=2$, then $a_2=5, a_3=7, \mathrm{\dots }, a_n=3+2(n-1)=2n+1$.  An isoperimetric series is defined as a series in which the ratio of each term to the previous term from term 2 onwards is the same constant, we call such a series an isoperimetric series and call this constant the common ratio of the isoperimetric series, which is usually denoted by the letter $q$$(q\ne 0)$. For example: $a_1=3, q=2, \mathrm{then} a_2=6, a_3=12, \mathrm{\dots }, a_n=3\times 2^{(n-1)}$.  (1) If the first term of an equidistant sequence $\left\{a_n\right\}$ is $d$ and the common difference is $a_n=a_1+(n-1)d$, then the general formula for this equidistant series is D. Write the general formula for an equidistant series based on the representation of the general formula for an equidistant series, labelling the meaning of each letter in the formula.  (2) The equal difference series has the following property: $a_n$, $a_m$ are any two terms in the equal difference series, and the relationship between $a_n$ and $a_m$ is $a_n=a_m+(n-m)d$. The proof process is as follows. $\begin{array}{l}\mathrm{left}=a_1+(n-1)d\\ \mathrm{right}=a_1+(m-1)d+(n-m)d=\mathrm{left}\end{array}$  Write the relationship between any two terms $a_n$ and $a_m$ in an isoperimetric series and justify your conclusion based on this property of the equidistant sequence.