Why does i² equal -1 in the wave function derivative?

[Another]

I don't understand "ψ (x,t) =Ae^i(px-Et)/h"

I understand dψ^2/dx^2 = i^2p^2/h^2

but why dψ^2/dx^2 = -p^2/h^2

"-" from
"i^2" missing
please help me

 

[hilbert2]

The imaginary unit $i$ is a number with property $i^2 = -1$. The only other number with same property is $-i$.

 

[Another]

The imaginary unit $i$ is a number with property $i^2 = -1$. The only other number with same property is $-i$.

thankful