Ответы на вопрос:
√18 cos² 7π/8 - √18 sin² 7π/8 = √18 (cos² 7π/8 - sin² 7π/8) = √18 * cos 7π/4 = √18 * cos 315° = √18 * √2/2 = √36/2 = 6/2 = 3 примечание: 1. cos²a - sin²a = cos2a 2. cos 315° = cos (270° + 45°) = cos270° cos45° - sin270° sin45° = 0*√2/2 - (-1)*√2/2 = √2/2 2.1 cos 270° = cos(180°+90°)=cos180° cos90° - sin180° sin90 ° = (-1)*0 - 0*1 = 0 2.2 sin 270° = sin(180°+90°)=sin180° cos90° + sin90° cos180 ° = 0*0 + 1*(-1) = -1
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