Câu 68
`P=cot1°.cot2°.cot3°...cot89°`
`=cot1°.tan1°.cot2°.tan2°...tan44°.cot44°.cot45°`
`=1`
Câu 69
`B=cos((3π)/2-alpha)+sin((3π)/2-alpha)+cos((3π)/2-alpha)-sin((3π)/2-alpha)`
`=sin((3π)/2-alpha)-sin((3π)/2-alpha)`
`sin((3π)/2-alpha)=-cosalpha`
`sin((3π)/2+alpha)=-cosalpha`
`=>B=-cosalpha+cosalpha=0`