更新时间2021-03-28 04:21:20
cos(π/4-x) > 0,故 -π/2 < π/4-x < π/2
-π/4 < x < 3π/4, -π/2 < 2x < 3π/2,-1 ≤ cos2x ≤ 1;
cos(π/4-x) = (√2/2)( cosx + sinx ) = 3/5;
( cosx + sinx )^2 = (3√2/5)^2
2sinxcosx = 18/25 - 1
sin2x = -7/25
cos2x = ±√[ 1 - (-7/25)^2 ] = ± 24/25
cos2x 可能等于 24/25 或 -24/25 。