devarshi-dt-logo

Question:

Evaluate: ∫₀ˣ x tanx secx + tanx dx

Solution:

LetI=∫x0xtanxsecx+tanxdxLet's first evaluate the integral and then substitute the limitsSo,I=∫xtanxsecx+tanxdx=∫xsinxcosx(1cosx+sinxcosx)dx=∫−ix(eix−e−ix)(eix+e−ix)(2eix+e−ix−i(eix−e−ix)eix+e−ix)dx. [Usingeix=cosx+isinx]=∫−ix(eix−e−ix)(eix+e−ix)(𕒶i2eix+e−ix−i(eix−e−ix)eix+e−ix)dx...[∵i2=𕒵]=∫−ix(eix−e−ix)−i(eix+e−ix)(2ieix+e−ix+(eix−e−ix)eix+e−ix)dx=∫x(eix−e−ix)(2i+(eix−e−ix))dx=∫x(e2ix𕒵)(2ieix+e2ix𕒵)dxTakeix=u⟹idx=du∴I=−∫u(e2u𕒵)(2ieu+e2u𕒵)duTakeeu=v⟹ln(v)=u,eudu=dvi.e.du=dveu=dvv∴I=−∫ln(v)(v2𕒵)v(2iv+v2𕒵)dv=−∫ln(v)((v+i)2𕒶iv)v(v+i)2dv=−(∫ln(v)vdv+∫ln(v)2i(v+i)2 dv)=−(I1+2iI2), whereI1is first integral andI2is second integralLet's evaluate second integralI2=∫ln(v)(v+i)2Letf=lnvandg′=1(v+i)2⟹f′=1v,g=𕒵v+iThenI2=∫f⋅g′=fg−∫f′⋅gI2=−ln(v)v+i+∫1v(v+i) dvI2=−ln(v)v+i+∫1v2(iv+1) dvPut(iv+1)=w⟹−iv2dv=dw∴I2=−ln(v)v+i−∫1i(w) dw=−ln(v)v+i−iln(w)∴I2=−ln(v)v+i−iln(iv+1)I1=∫ln(v)vdvTakeln(v)=w⟹1vdv=dw∴I1=∫w dw∴I1=w22∴I1=(ln(v))22∴I=−((ln(v))22𕒶iln(v)v+i𕒶i×iln(iv+1))∴I=−(ln(v))22𕒶iln(v)v+i+2i2ln(iv+1)+C=−(ln(v))22𕒶iln(v)v+i𕒶ln(iv+1)+C...[∵i2=𕒵]=−(ln(eu))22𕒶iln(eu)eu+i𕒶ln(ieu+1)+C=−(ln(eix))22𕒶iln(eix)eix+i𕒶ln(ieix+1)+C=−(ix)22𕒶i(ix)eix+i𕒶ln(ie−ix+1)+C..[∵lne=1]=−i2x22𕒶i2xeix+i𕒶ln(ie−ix+1)+C=x22+2xeix+i𕒶ln|ie−ix+1|+C