a) Given that

R1 = 500Ω

R2 = 1000Ω

From the above circuit diagram,

R = R1 + R2

= 500 + 1000

=1500Ω

b) Given that

R1 = 2Ω

R2 = 2Ω

From the given circuit diagram,

1/R = 1/R1 + 1/R2

1/R = ½ + ½

R = 1Ω