<s>[INST] Calculate for $(1+\tan 20^\circ)(1+\tan 25^\circ)$. Help me with my works I have no idea what I am doing here, 
I started with $\tan 20^\circ=\tan(45^\circ-25^\circ)=(1-\tan 25^\circ)/(1+\tan 25^\circ)$ 
I am sure the work I have shown so far are ok, but how do you get $1+\tan 20^\circ=2/(1+\tan 25^\circ)$ from that?
 [/INST] $1 = \tan(45^\circ) = \tan(20^\circ+25^\circ) = \frac{ \tan 20^\circ + \tan 25^\circ}{1- \tan 25^\circ \tan 20^\circ} $
so
$2 = 1+ \tan 20^\circ + \tan 25^\circ + \tan 20^\circ \tan 25^\circ = (1+ \tan 20^\circ)(1+ \tan 25^\circ)$. 
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