sin cos tan cot 1 sec^4 - sec² 化简成 1/cot4 + 1/cot²2 sec/cos

sin^2+cos^2=1
1.sec^4-sec^2=(1/cos^4)-1/(cos^2)=(sin^2)/(cos^4)
=[(sin^2)*(sin^2+cos^2)]/cos^4=1/cot4 + 1/cot²
2.sec/cos - tan/cot=1/(cos^2)-(sin^2)/(cos^2)=cos^2/cos^2=1
3.(1-2cos²)/(sincos) =(sin^2+cos^2-2cos^)/(sincos)=(sin^2-cos^2)/(sincos)=tan - cot

1.sec^4 - sec² =1/cos^4-1/cos^2=
（1-cos^2）/cos^4=sin^2/cos^4=(sin^4+sin^2-sin^4)/cos^4
=sin^4/cos^4+(sin^2-sin^4)/cos^4
=tg^4+sin^2*cos^2/cos^4
=1/cot^4+sin^2/cos^2=1/cot^4 + 1/co...