Control Systems
In frequency response analysis of second order system, the error at corner frequency is

 -20 log2ξ
 -10 logξ
 10 logξ
 20 log2ξ

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Control Systems
A cascade of three linear time invariant systems is causal and unstable. From this we conclude that

 at least one system is causal and all systems are unstable.
 each system in the cascade is individually caused and unstable.
 the majority are unstable and the majority are causal.
 at least one system is unstable and at least one system is causal.

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