The values of b for which the function $f(x)=cos\, x +bx + a $ decreases on R are
Answer & explanation
Correct answer: option 3
The correct answer is Option (2) → (-∞, 1]
$f(x)=\cos x +bx + a$
$f'(x)=-\sin x+b<0$
⇒ for $b∈(-∞,-1]$
$-\sin x+b<0$
$f(x)$ → decreasing