>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?