Solve $${{{d^4}y} \over {d{x^4}}} - y = 15\,\cos \,\,2x$$

$${\left( {s + 1} \right)^{ - 2}}$$ is laplace transform of

The Laplace Transform of a unit step function $${u_a}\left( t \right),$$ defined as
$$\matrix{ {{u_a}\left( t \right) = 0} & {for\,\,\,t < a\,} \cr { = 1} & {for\,\,\,t > a,} \cr } $$ is

The continuous function $$f(x, y)$$ is said to have saddle point at $$(a, b)$$ if