Помоги те найти общее сопротивление с любыми числами

1)
r=r1+r2+r1*r2*r3*r4/(r2*r3*r4+r1*r3*r4+r1*r2*r4+r1*r2*r3)
2)
r=r1*r2/(r1+r2) + r3*r4*r5/(r4*r5+r3*r5+r3*r4)
3)
r=r4+r1*r2*r3/(r2*r3+r1*r3+r1*r2)
4)
r=(r3*r4*r5/(r4*r5+r3*r5+r3*r4)+r6)*(r1*r2/(r1+r2)) / (r3*r4*r5/(r4*r5+r3*r5+r3*r4)+r6) + (r1*r2/(r1+r2))
или в "человечей" записи
$\frac{ (\frac{r3*r4*r5}{r4*r5+r3*r5+r3*r4} +r6)( \frac{r1*r2}{r1+r2})}{\frac{r3*r4*r5}{r4*r5+r3*r5+r3*r4} +r6+ \frac{r1*r2}{r1+r2} }$