Three vectors are linearly independent if the wronskian in non-zero
Here the wronskian is
| 1 ex + e-x ex - e-x |
| 0 ex - e-x ex+ e-x |
| 0 ex + e-x ex - e-x |
from Principles of determinant calculation
Expanding along First column
1 * | ex - e-x ex+ e-x |
= | ex+ e-x ex - e-x |
= -4
Which is not equal to zero
Hence the vectors are linearly independent
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