1. The value of cos² x + cos² y – 2cos x × cos y × cos (x + y) is

2. If cos a + 2cos b + cos c = 2 then a, b, c are in

3. If the angles of a triangle be in the ratio 1 : 4 : 5, then the ratio of the greatest side to the smallest side is

4. If a×cos x + b × cos x = c, then the value of (a × sin x – b²cos x)² is

5. The value of cos 5π is

6. In a triangle ABC, cosec A (sin B cos C + cos B sin C) equals

7. The value of cos 180° is

8. The perimeter of a triangle ABC is 6 times the arithmetic mean of the sines of its angles. If the side b is 2, then the angle B is

9. If 3 × tan(x – 15) = tan(x + 15), then the value of x is

10. If the sides of a triangle are 13, 7, 8 the greatest angle of the triangle is