JEE Mains Model Paper 10

Differentiating both sides with respect to x,

cos5x+cos4x

1−2cos3x

= -2Acos2x + Bcosx
x = 0 →

1+1

1−2

= -2A + B → -2A + B = -2
x =

π

2

→

0+1

1−2(0)

= -2A (-1) + 0 → A =

1

2

∴ B = -2 + 2A = -2 + 1 = -1
∴ A + B =

1

2

+ (-1) =

−1

2