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so basically what I had in my head was complete insanity? .. why was I thinkin of ln?
Also, why are we "supposing" that x=tan theta? (I get that this is some ancient dervied thing.. but could x have been supposed as anything?)
So that we can cancel out sec² Ө even before we begin integrating!
1 + tan² Ө = sec² Ө ; ∫1/(1 + x²) dx ; dx = sec² Ө . dӨ............... [do you see it now? ]
∫1/(1 + x²) dx
suppose x = tan Ө so Ө = tan^(-1) x
so 1 + x² = 1 + tan² Ө = sec² Ө
and dx/dӨ = sec² Ө
dx = sec² Ө . dӨ
So ∫1/(1 + x²) dx = ∫(1/sec² Ө) . sec² Ө dӨ
= ∫ 1 dӨ
= Ө + k
= tan^(-1) x + k