Prove that: tan13A-tan9A-tan4A=tan13A. tan9A. tan4A

Prove that: tan13A-tan9A tan4A=tan13A tan9A tan4A For this equation, we will use the formula: I) tan 2a= tan(a + a) and, II) tan (a + b)= (tan a+ tan b)/(1-tan a tan b) From (I), we can say tan 13A= tan (9A+4A) but, tan(9A+4A)=(tan 9A + tan 4A)/ (1-tan 9Atan 4A) Therefore tan13A=(tan 9A+tan4A)