back substitution的用法和樣例：
例句
- We see immediately that after substitution the real part of each term is 0.
我們立即看出，在代換之後，每項的實數皆為零。 - In fact, we could make the substitution and still wind up with the correct eigenfunctions and eigenvalues.
事實上，我們可以做這種代換而仍能繞彎得出正確的本徵函數和本徵值。 - The procedure for solving the Eqs is called back substitution.
解方程組的這一步驟叫回代。
