 Solution

Write the initial triangular pulse as the function $h$, extended by zero to negative arguments:At $t=0$, the initial displacement and zero velocity give $f+g=h$ and $-c{f}^{\prime }+c{g}^{\prime }=0$ on the positive half-line. Fix the harmless opposite additive constants by taking both travelling waves to vanish at positive infinity. ThenThe second formula follows from the fixed-end boundary condition $f(-ct)+g(ct)=0$. Thus $f$ is one-half of the odd extension $H$ of the initial displacement, andThis is the odd-extension solution for a semi-infinite fixed string: the reflected displacement reverses sign. Evaluating the travelling profiles givesThe first profile is a positive triangle supported on $[L,3L]$, with peak $a/2$ at $2L$. The second has a negative triangle on $[0,2L]$ with minimum $-a/2$ at $L$, and a positive triangle on $[2L,4L]$ with maximum $a/2$ at $3L$.