Given: f(x)=x2−6x−16x
Now differentiating the above function we get,
⇒f′(x)=(x2−6x−16)2(x2−6x−16)⋅1−x⋅(2x−6)
⇒f′(x)=(x2−6x−16)2−x2−16
⇒f′(x)<0...(i)
Now, x2−6x−16=0
⇒x2−8x+2x−16=0
⇒(x+2)(x−8)=0
⇒x=8,−2...(ii)
Using (i)and(ii),
So, the function is always decreasing inx∈(−∞,−2)∪(−2,8)∪(8,∞).