Gate Exam

Consider the following relations:
R1 (a,b) iff (a+b) is even over the set of integers
R2 (a,b) iff (a+b) is odd over the set of integers
R3 (a,b) iff a.b > 0 over the set of non-zero rational numbers
R4 (a,b) iff a + bi 2 over the set of natural numbers
Which of the following statements is correct?
(a) Ri and R2 are equivalence relations, R3 and R4 are not
(b) Ri and R3 are equivalence relations, R2 and R4 are not
(c) Ri and R4 are equivalence relations, R2 and R3 are not
(d) Ri, R2, R3 and R4 are all equivalence relations

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Gate Other Question
Consider the following statements:
S1: The sum of two singular n x n matrices may be non-singular
S2: The sum of two n x n non-singular matrices may be singular.
Which of the following statements is correct?
(a) 51 and S2 are both true
(b) 51 is true, S2 is false
(c) 51 is false, S2 is true
(d) 51 and S2 are both false Consider two well-formed formulas in prepositional logic
El: P - iP
E2: (Pi - iP)v(--iPiP)
Which of the following statements is correct?
(a) Fl is satisfiable, F2 is valid
(b) Fl unsatisfiable, F2 is satisfiable
(c) Fl is unsatisfiable, F2 is valid
(d) Fl and F2 are both satisfiable