Prove the following question.
∫π402 tan3x dx=1−log 2.
Let I=∫π402 tan3x dx=2∫π40tan2x.tan x dx=2∫π40(sec2x−1)tan x dx [∵ 1+tan2x=sec2x]=2[∫π40sec2x tan x dx−∫π40tan x dx]=2∫π40(tan x)sec2x dx−2[−log |cos x|]π40 [Let, ∵ I1=∫(tan x)sec2x dx put tan x=t⇒ sec2x dx=dt ∴ I1=∫t dt=t22=tant2x2]=2[tan2x2]π40+2[log∣∣cos π4∣∣−log |cos 0|]=tan2(π4)−0+2 log (1√2)−log 1=1+2 log x−12−0 (∵ log 1=0)=1−2×12 log 2=1−log 2. hence proved.