Magnitude just means "size" or "bigness" - so it can be used in many different situations.
Pythagoras' theorem just helps us to calculate magnitude in some situations:
If you imagine a graph with x going across and y going upwards, you can draw a line from the centre (where the axes cross at 0,0) to any point you like... for example the point 4,3 (x=4, y=3).
So if you then draw another line along the x axis to 4, and another up to the end of your line, you have drawn a triangle, and you can use pythagoras to find the length (the magnitude) of your line
magnitude^{2} = x^{2} + y^{2}
= 4^{2} + 3^{2}
= 16 + 9 = 25
so magnitude in this case is sqrt (25) = 5
you could use this to calculate the size of a piece of wood you would need on a gate, or something much more advanced.
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