Since
f(x)=โซx2โ9x+20x+3โdx
we have
fโฒ(x)=x2โ9x+20x+3โ
Now factorise the denominator:
x2โ9x+20=(xโ4)(xโ5)
So,
fโฒ(x)=(xโ4)(xโ5)x+3โ
The function is not defined at x=4 and x=5.
Now check the sign of fโฒ(x) on the intervals formed by โ3,4,5.
For x<โ3:
x+3<0,(xโ4)(xโ5)>0
So,
fโฒ(x)<0
Hence f(x) decreases on (โโ,โ3].
For โ3<x<4:
x+3>0,(xโ4)(xโ5)>0
So,
fโฒ(x)>0
Hence f(x) increases on (โ3,4).
For 4<x<5:
x+3>0,(xโ4)(xโ5)<0
So,
fโฒ(x)<0
Hence f(x) decreases on (4,5).
For x>5:
x+3>0,(xโ4)(xโ5)>0
So,
fโฒ(x)>0
Hence f(x) increases on (5,โ).
So f(x) decreases on
(โโ,โ3]โช(4,5)
Correct option:
Optionย Cโ