1-2sin²75°

1-2sin²75°
1-2sin²75°

1-2sin²75°
1-2sin^275°
=1-2sin^2(180°+95°)
=1-2sin^2(95°)
=1-2cos^2(5°)
=sin^2(5°)-cos^2(5°)
=-cos10°

-1+根号3

1-2sin² 75°=cos(2×75)=cos150=cos(180-30)=cos180cos30+sin180sin30=-cos30=-√3/2