The knife is to dull to cut anything.
To use the word "dull" in a sentence, you could say: "The lecture was so dull that I struggled to stay awake."
The quick-witted boy was by no means dull.
This is a very dull answer to your question.
Flat paint leaves a dull finish.
This dull finish just needs a little buffing.
you can say my family and i went to the national forest and found a beautiful, but dull beech.
"The sky is looking dull today." "Talking to Joe is really dull." "Mary finds maths dull."
My mother had bought me a new bucket of 'Nipton' paint to decorate my room, but I didn't like the colour because it was too dull.
The actors did their best but the dialog was so dull that they couldn't save the play.
The envelope and its portentous contents fell onto the mat with a dull thud.
The missing word in the sentence is "blunt." The analogy compares opposites: "separate" is the opposite of "together," just as "sharp" is the opposite of "blunt," which is often considered synonymous with "dull." Thus, the complete analogy is "Separate is to together as sharp is to blunt."
The professor's lectures are often dull and uninteresting. Without highlights, my hair is lifeless and uninteresting. A good public speaker will make an uninteresting topic exciting for the audience.