Answer:
1 + tan²A = sec²A = 1/cos²A
2tan(X/2)/(1+tan²(X/2))
= 2tan(X/2)cos²(X/2)
= 2[sin(X/2)÷cos(X/2)]cos²(X/2)
= 2sin(X/2)cos(X/2)
= sin(X)
Because sin(2A) = 2sin(A)cos(A)
