what is the interval for a straight line (linear function)?

y = 0.5x + 4
 
what is the interval at which the function increasing( or decreasing)?
 
 
is the interval (-∞ < x < ∞) ?
 
[-∞, ∞]
 
 

the line has a positive slope.  therefore, as x increases from -∞ to ∞, the linear function increases.
 
also, the interval is (-∞, ∞).  we use parentheses rather than brackets.  this is because infinity is not a defined number.  it can be a very large number or a very small number that we do not know of exactly.  the number increases or decreases continuously.