I'm part way through a question, just need help at the end.

I found the characteristic equation for a fourth order DE with constant co-efficents (and homogeneous) was r^4 - 2r^2 +1=0 so the roots are r = 1, -1 each with multiplicity 2.

so $\displaystyle y=e^x(A+Bx)+e^{-x}(B+Dx)$ I need to get it in terms of sinh and cosh. Thanks