Question 969675
verify ((tan(x+y)-tany)/(1 +tan(x+y)tany))=tanx
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{{{Identity: tan(a-b)=(tana-tanb)/(1+tana*tanb)}}}
start with left side:
{{{((tan(x+y)-tany))/(1 +tan(x+y)tany)=tan(x+y-y)=tanx}}}
verified: left side=right side