Question

prove that: tan9A-tan5A-tan4a=tan9a.tan5A.tan4a

Answer 1

9A = 4A +5A
Take both side Tan.


Answer see in attachment 《●》

Answer 2

9A = 5A + 4A
take both sides, tan
tan9A = tan(5A + 4A)
use, formula,

tan(C + D) = (tanC + tanD)/(1 - tanC.tanD)

tan9A = (tan5A + tan4A)/(1 - tan5A.tan4A)

tan9A(1 - tan5A tan4A) = tan5A + tan4A

tan9A - tan9A.tan5A.tan4A = tan5A +tan4A

tan9A - tan5A - tan4A = tan9A.tan5A.tan4A
hence proved //