18. Simplify: $\frac{2 \tan x}{1+\tan ^{2} x}$
\(\boxed{\frac{2 \tan x}{\sec^2 x}}\)
Step 1 :Given expression: \(\frac{2 \tan x}{1+\tan ^{2} x}\)
Step 2 :Use the identity: \(\tan^2 x + 1 = \sec^2 x\)
Step 3 :Replace \(\tan^2 x\) with \(\sec^2 x - 1\): \(\frac{2 \tan x}{\sec^2 x - 1 + 1}\)
Step 4 :Simplify the expression: \(\frac{2 \tan x}{\sec^2 x}\)
Step 5 :\(\boxed{\frac{2 \tan x}{\sec^2 x}}\)