Bonjour,
1a) 1/R = 1/R1 + 1/R2
1/R = 1/2 + 1/5
1/R = (1 x 5)/(2 x 5) + (1 x 2)/(5 x 2)
1/R = 5/10 + 2/10
1/R = 7/10
R = 10/7 Ω
1b) 1/R = 1/4 + 1/13
1/R = 13/52 + 4/52
1/R = 17/52
R = 52/17 Ω
1c) 1/R = 1/11 + 1/13
1/R = 13/143 + 11/143
1/R = 24/142 Ω
2a) 1/R = 1/R1 + 1/R2
1/R - 1/R2 = 1/R1
1/R1 = 1/6 - 1/24
1/R1 = (1 x 4)/(6 x 4) - 1/24
1/R1 = 4/24 - 1/24
1/R1 = 3/24
R1 = 24/3 = 8 Ω
2b) 1/R1 = 1/1,1 - 1/1,7
1/R1 = 10/11 - 10/17
1/R1 = 170/187 - 110/187
1/R1 = 60/187 Ω