I understood that even though W is non zero, y1 and y2 do not form fundamental solutions because ehy don't satisfy Thm3.2.5.
But after that, how do we pick y3 and y4?
For y3, as c1 and c2 are 1/2, so 1/2e^t + 1/2e^-t makes sense, but for y4 how is there a negative sign? (1/2e^t + 1/2e^-t)
