Question

How are photosynthesis and cellular respiration opposite in terms of carbon dioxide

Answer 1

Photosynthesis is the process by which plants take in (use) Carbon-dioxide and water to make food (starch). 
                                  Photosynthesis: 
$CO_{2} + H_{2} 0 +sunlight ----> C_{6}H_{12}O_{6}+O_{2}$
                                                
Cellular respiration is the process in which cells use oxygen to break down starch into energy molecules and give out Carbon-dioxide.
                                Cellular respiration:
 $C_{6}H_{12}O_{6}+O_{2}----->CO_{2} + H_{2} 0 +energy$

So, in Photosynthesis, carbon-dioxide is taken in (used) and in cellular respiration, carbon-dioxide is thrown out (waste product)

Answer 2

Things that use Photosynthesis use carbon dioxide and release oxygen (like plants)
Things that use cellular respiration use oxygen and release carbon dioxide (like humans)