a) I=E/R+r
I=E/R+r1
1.2=12/7+r1
1.2(7+r1)=12
8.4+1.2r1=12
1.2r1=12-8.4
1.2r1=3.6
r1=3.6/1.2
r1=3
1.2=12/6+r2
1.2(6+r2)=12
7.2+1.2r2=12
1.2r2=12-7.2
1.2r2=4.8
r2=4.8/1.2
r2=4
1.2=12/3+r3
1.2(3+r3)=12
3.6+1.2r3=12
1.2r3=12-3.6
1.2r3=8.4
r3=8.4/1.2
r3=7
b) I2= E/R2+r2
I2=12/6+3
I2=12/9
I2=1.3
c)r1+r2+r3=3+4+7=12
d) I=E2/R2+r2
1.2=E2/6+4
1.2x10=E2
E2=12
