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I was reading a trig book in which the author had mentioned a trick to jump from $(\theta+90^\circ)$ to $(\theta-90^\circ)$ formulas. I understood it for Sine and Cosine but couldn't for Tangent.
Here is the $(\theta+90^\circ)$ formula…
He mentioned to replace $\theta$ in the above formula by $(\theta-90^\circ$ to obtain the desired $(\theta-90^\circ)$ formula. I did the same and ended up getting this.
which is wrong according to the book.
It says the answer should be…
I do not where am I going wrong. What is incorrect in my approach? What is the flaw? Please help me track it down.