To prove tan10+tan35+tan10.tan35=1 we have to known that tan45=1
=> 1=tan45 => 1=tan(35+10)
replace tan(35+10) with tan(A+B) formula which is tanA+tanB/1-tanA.tanB so equation becomes => 1=tan35+tan10/1-tan35.tan10 => 1-tan35.tan10=tan35+tan10 => tan35+tan10=1-tan35.tan10 =>tan35+tan10+tan35.tan10=1
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