27) Đáp án D
A=tan1°.tan2°....tan88°.tan89°
=tan1°.tan89°.(tan2°.tan88°)....(tan 44°.tan46°).tan45°
=tan1°.cot1°.(tan2°.cot2°)...(tan 44°.cot44°).tan45°
=1.1...1.1
=1
28) Đáp án C
sinx=2/5
sin^2x+cos^2 x=1
=>cos^2 x=1-sin^2 x=1-(2/5)^2=21/25
=>cosx=±√21 /5
Vì 90°<x<180°
=>cosx<0
=>cosx=-√21 /5