(cos 15° + sin 15°)² = cos²15° + 2cos15°sin15° + sin²15° = (cos²15° + sin²15°) + 2cos15°sin15° = 1 + sin(2∙15°) = 1 + sin30° = 1 + 1/2 = 3/2.
Ответ: 3/2