Which of the following differential equations has y=c1ex+c2e−x as the general solution?
(a) d2ydx2+y=0
(b) d2ydx2−y=0
(c) d2ydx2+1=0
(d) d2ydx2−1=0
Given, general solution is y=c1ex+c2e−x ...(i)
On differentiating twice w.r.t. x, we get y′=c1ex+c2e−x(−1)
Again, differentiating w.r.t. x, we get
y′′=c1ex−c2e−x(−1)⇒y′′=c1ex+c2e−x⇒y′′=y⇒y′′−y=0
Which is the required differential equation of the given general solution. Replacing y' by dydx and y'' by d2ydx2. Hence, option (b) is correct option.