>We need to know how much angle B changes at the change of A

This sounds like one of two things:

* the **rate of change** of B depending on A

* called **dB/dA**

* for example, when A=30° is increasing a tiny amount, then B is decreasing by 5 times this tiny amount

* the **increase** of B depending on A

* called **ΔB**

* for example, when A=30° is increasing to 35°, then B is decreasing by 110°

>certain length x

So x is a constant? You choose/measure/calculate it once, and then it keeps its value when A and B change, right?