isocost - line showing all combos of labor and capital bought for a given cost  

• rental rate - r, given to certain equipment to quantify capital
• C = wL + rK
  • MPL/MPK = w/r = MRTS

cost-minimization - finding lowest isocost curve intersecting given isoquant  

• found at point of tangency between isoquant and isocost
• expansion path - curve passing through cost minimization points (points of tangency between isocost and isoquant)
  • long-run total cost curve derived from expansion path
  • shows combinations of labor/capital that the firm needs at each output level, will increase if labor/capital increase w/ output