If i got properly your question here is the code you need:
etha = 0.2:0.2:1;
plot(etha,1./etha - 1);
You have to use the ./ operation instead of the / ones because you are dividing a scalar by a vector. The ./ divide the scalar by each of the vector elements.
This is a really basic question, so i suggest you to read the plot matlab documentation and do the basic tutorial mathworks provide http://it.mathworks.com/support/learn-with-matlab-tutorials.html.
It will be useful to understand better how matlab works! Hope my answer helped you.
