cos^2（π/4-α）-sin^2（π/4-α）
={cos（π/4-α）+sin（π/4-α）} {cos（π/4-α）-sin（π/4-α) }
=根號(hào)2{cos π/4 cos（π/4-α）+sinπ/4 sin（π/4-α）} 根號(hào)2{cosπ/4 cos（π/4-α）-sinπ/4 sin（π/4-α) }
=2 cos[π/4-(π/4-α）] {cos[π/4+(π/4-α)]
=2 cosα cos(π/2-α)
=2 cosα sinα
=sin2α