Final answer:
Two AC waveforms represented by a vector diagram at a right angle are 90 degrees (π/2 radians) out of phase, known as quadrature, and not in pure destructive interference which happens at 180 degrees.
Step-by-step explanation:
If the vector diagram of two AC waveforms is at a right angle, then the waves are 90 degrees or π/2 radians out of phase. When two sinusoidal waves are 90 degrees out of phase, they reach their maximum and minimum values at different times. This is often referred to as quadrature in the context of AC waveforms.
When waves are 180 degrees or π radians out of phase, such as the scenarios described in the figures provided, they result in pure destructive interference, where the waves cancel each other's amplitude and produce a resulting wave equal to zero. However, this is a different scenario from being 90 degrees out of phase, which results typically in the formation of a combined waveform that can be seen in systems exhibiting orthogonal crossing waves, like in some signal processing applications.