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If the function f(x)=3 cos |x| -6 ax +b increases for all `x in R` then the range of value of a given byA. `(-1/2, oo)`B. `(-oo,-1//2)`C. `(-oo,-2)`D. `(-2 ,oo)` |
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Answer» Correct Answer - B We have f(x)=3 cos |x|-6ax +b `rArr f(x) =-3 sin x -6ax +b` `rArr f(x) =3 sin -6 ax + b` `[ because cos |x| =cos x for all x ]` `rArr f(x)=-3 sinx -6a` For f(x) to be increasing on R we must have `f(x) gt 0 "for all" n in R` `rArr -3 sin x-6 a gt 0 "for all"x in R` `rArr sin x+2a lt 0 "for all x in R` `rArr sin x lt -2a " for all " x in R` `rArr 1 lt - 2a " "[ because "`Max .value of sin x is 1] `rArr a lt -1/2` `rArr a in (-oo,-1 //2)` |
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