Simplification Made to Gain Equation
Since \$g_{m1}r_{o1} \gg 1\$, we can neglect the 1 inside bracket. Then, 
$$R=g_{m1}r_{o2}r_{o1}+r_{o1}=(g_{m1}r_{o2}+1) r_{o1}$$

Since \$g_{m1}r_{o2} \gg 1\$,

$$R= g_{m1}r_{o2}r_{o1}$$