Given,
A={1,2,3,4,5},B={4,5,6,7,8},C={7,8,9,10,11} and D={10,11,12,13,14}.
For
A∪B∪C
A∪B∪C ={1,2,3,4,5} ∪ {4,5,6,7,8}
A∪B∪C ={1,2,3,4,5,6,7,8}
Now,
A∪B∪C=(A∪B)∪C
A∪B∪C={1,2,3,4,5,6,7,8}∪{7,8,9,10,11}
Therefore, A∪B∪C={1,2,3,4,5,6,7,8,9,10,11}
A∪B∪C={1,2,3,4,5,6,7,8,9,10,11}