" 13.Prove that: "(tan69^(@)+tan66^(@))/(1-tan69^(@)tan66^(@))=-1

Prove that :(tan69^@+tan 66^@)/(1-tan 69^@tan66^@)=-1

Simplify be reducing to a single term : (tan 69^(@) + tan 66^(@))/(1 - tan 69^(@) tan 66 ^(@))

Prove that (i) "cos " 15^(@) - " sin " 15^(@) = (1)/(sqrt(2)) (ii) " cot " 105^(@) - " tan " 105^(@) =2sqrt(3) (iii) (tan 69^(@) + tan 66^(@))/(1-tan 69^(@) tan 66^(@)) =-1

Prove that:(i) "tan"69^(@)+"tan"66^(@)+1="tan"69^(@) "tan"66^(@) (ii) (cos13^(@)+"sin"13^(@))/(cos13^(@)-"sin"13^(@))="tan"58^(@)

Prove that : tan 69^@ + tan 66^@+ 1 = tan 69^@ tan 66^@ .

Prove that: tan10^(@) tan20^(@)tan70^(@)tan80^(@)=1

Prove that: tan10^(@) tan20^(@)tan70^(@)tan80^(@)=1

Prove that: tan20^(@)tan30^(@)tan40^(@)tan80^(@)=1

If tan 25^(@)=a , prove that (tan 155^(@)-tan115^(@))/(1+tan155^(@)tan115^(@))=(1-a^(2))/(2a) .

Prove that: tan6^(0)tan42^(0)tan66^(@)tan78^(@)=1