There are many formulae that a student has to know and sometimes remember for his studies, and applications.
How then can he capture all these necessary formulae for usage?
One way is to understand the principles and derive those required later on for usage.
In trigonometry, you will come across sine of two angles, sin (A + B).
You have memorised sin (A + B) as "sinA cosB + cosA sinB".
However, when you further need to know sin 2A, what then?
If you know and understand the principles of the sin(A + B), you can easily move on to derive the sin2A.
Since you know sin (A + B), you can equate A = B, to allow you to get the sin(A + A).
As sin (A + A) = sin2A, you will then have no problem achieving
sin(A +A) = sinA cosA + cosA sinA ==> sin 2A = 2sinAcosA
There, you have obtained another formula without the need to memorise it.
Thus mastering the sin (A + B) principles or its equivalent, can allow you to expand your knowledge further.
You see the benefits now?
I have shown only the trigonometry part in math learning, but, you can appreciate that it applies to any other topics as well.
Seek, therefore to handle principles of math well as it will serve you good in the long run.
:-) ..... :-) Forever liking mathematics. Maths is interesting!