is it true that probability of occurrence of event A or B =P(A+B) =P(A)+P(B)

Dear Student,
Given statement is not true .
Probability of occurence of A or B is denoted by $P(A\cup B)$
and probability of occurence of A or B or atleast one of A or B$P(A\cup B)=P\left(A\right)+P\left(B\right)-P\left(A\cap B\right)$

if events are mutually exclusive then $P(A\cap B)=0$
now $P(A\cup B)=P\left(A\right)+P\left(B\right)$
hope this will help
enjoy