use the product rule
#f(x)=color(red)(u)color(blue)(v)=>f'(x)=color(blue)(v)color(red)(u')+color(red)(u)color(blue)(v')#
#f(x)=color(red)(5secx)color(blue)(tanx)#
#color(red)(u=5secx=>u'=5secxtanx#
#color(blue)(v=tanx=>v'=sec^2x#
#:.f'(x)=color(blue)(tanx)color(red)(5secxtanx)+color(red)(5secx)color(blue)(sec^2x#
#f'(x)=5secxtan^2x+5sec^3x#
#f'(x)=5secx(tan^2x+sec^2x)#
#f'(x)=5secx(tan^2x+1+tan^2x))#
#f'(x)=5secx(2tan^2x+1)=10secxtan^2x+5secx#