cosα-1/cosα=-2
(cosα)^2-1=-2cosα
(cosα)^2+2cosα-1=0
(cosα)^2+2cosα+1-2=0
(cosα+1)^2-2=0
(cosα+1-√2)(cosα+1+√2)=0
cosα=√2-1或cosα=-1-√2(舍去)
cosα+1/cosα
=√2-1+1/(√2-1)
=√2-1+(√2+1)/[(√2-1)(√2+1)]
=√2-1+√2+1
=2√2
cosα-1/cosα=-2
(cosα)^2-1=-2cosα
(cosα)^2+2cosα-1=0
(cosα)^2+2cosα+1-2=0
(cosα+1)^2-2=0
(cosα+1-√2)(cosα+1+√2)=0
cosα=√2-1或cosα=-1-√2(舍去)
cosα+1/cosα
=√2-1+1/(√2-1)
=√2-1+(√2+1)/[(√2-1)(√2+1)]
=√2-1+√2+1
=2√2