分子分母同除以cos2x
1/(1+sinx*cosx)=[1/cos2x]/[ 1/cos2x +tanx]
而1/cos2x=(sin2x+ cos2x)/ cos2x=tan2x+1=4+1=5
1/(1+sinx*cosx)=[1/cos2x]/[ 1/cos2x +tanx]=5/(5+2)=5/7