Let L be the set of all lines in XY-plane and R be the relation in L defined as R = {(L1, L2): L1 is parallel to L2}. Show that R is an equivalence relation. Find the set of all lines related to the line y = 2x + 4.
We have, L is the set of lines.
R = {(L1, L2) : L1 is parallel to L2} be a relation on L
Now,
Proof :
To prove that relation is equivalence, we need to prove that it is reflexive, symmetric and transitive.
Reflexivity : For Reflexivity, we need to prove that-
(a, a) ∈ R
Since a line is always parallel to itself.
∴ (L1, L2) ∈ R
⇒ R is reflexive
Symmetric : For Symmetric, we need to prove that-
If (a, b) ∈ R, then (b, a) ∈ R
Let L1, L2∈ L and (L1, L2) ∈ R
⇒ L1 is parallel to L2
⇒ L2 is parallel to L1
⇒ (L1, L2) ∈ R
⇒ R is symmetric
Transitive: For Transitivity, we need to prove that-
If (a, b) ∈ R and (b, c) ∈ R, then (a, c) ∈ R
Let L1, L2 and L3∈ L such that (L1, L2) ∈ R and (L2, L3) ∈ R
⇒ L1 is parallel to L2 and L2 is parallel to L3
⇒ L1 is parallel to L3
⇒ (L1, L3) ∈ R
⇒ R is transitive
Since, R is reflexive, symmetric and transitive, so R is an equivalence relation.
And, the set of lines parallel to the line y = 2x + 4 is y = 2x + c For all c ∈ R
where R is the set of real numbers.