8. How many different codes can be formed from the lower-case letters (a - z) of the English alphabet, if a code consists of seventeen distinct letters, with different orders counted as different codes.(10%)

5. Solve the telegraph equation for 0 ＜x ＜L, t ＞ 0. A and B are positive constants. The boundary conditions are u(0,t)=u(L,t)=0 for t ≥ 0, and the initial conditions are Assume that A2L2 ＜4(BL2+c2π2) (20%)