Given relation
#secalpha=secbetasecgamma+tanbetatangamma#
#=>secalpha-secbetasecgamma=tanbetatangamma#
#color(green)("Squaring both sides")#
#=>(secalpha-secbetasecgamma)^2=tan^2betatan^2gamma#
#=>sec^2alpha+sec^2betasec^2gamma-2secalphasecbetasecgamma=tan^2betatan^2gamma#
#=>-2secalphasecbetasecgamma=-sec^2alpha+tan^2betatan^2gamma-sec^2betasec^2gamma#
#=>-2secalphasecbetasecgamma=-sec^2alpha+(sec^2beta-1)(sec^2gamma-1) -sec^2betasec^2gamma#
#=>-2secalphasecbetasecgamma=-sec^2alpha+cancel(sec^2betasec^2gamma)+1-sec^2gamma-sec^2beta-cancel(sec^2betasec^2gamma)#
#=>sec^2beta-2secalphasecbetasecgamma=-sec^2alpha+1-sec^2gamma#
#color(blue)("Adding "(sec^2gammasec^2alpha)" both sides "#
#=>sec^2beta-2secalphasecbetasecgamma+sec^2gammasec^2alpha=sec^2gammasec^2alpha-sec^2alpha+1-sec^2gamma#
#=>(secbeta-secgammasecalpha)^2=sec^2alpha(sec^2gamma-1)-1(sec^2gamma-1)#
#=>(secbeta-secgammasecalpha)^2=(sec^2gamma-1)(sec^2alpha-1)#
#=>(secbeta-secgammasecalpha)^2=tan^2gammatan^2alpha#
#=>(secbeta-secgammasecalpha)=+-sqrt(tan^2gammatan^2alpha)#
#=>secbeta-secgammasecalpha=+-tangammatanalpha#
#=>color(BLUE)(secbeta=secgammasecalpha+-tangammatanalpha)#
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