From where I have highlighted in the picture, it says LAC AND LMC curves happen to be linear, can there be cases when they are not linear, eg give examples please, thanks

Q) .s 4.1 s -\":'. a e W LAC = LMC /unit of output Slope = LMC = LAC FIGURE 11.17 The LTC, LMC and LAC Curves with Constant Returns to Scale (a) With constant returns, long-run total cost is strictly proportional to output. (b) Long-run marginal cost is constant and equal to long-run average cost. When the production function has decreasing returns to scale, a given proportional increase in output requires a greater proportional increase in all inputs and hence a greater proportional increase in costs. The LTC, LMC and LAC curves for a production function with decreasing returns to scale are shown in EiguLeJlJ. For the particular LTC curve shown in Figure 11,181g), the associated LAC and LMC curves happen to be linear, as in EiguLLllJBLb), but this need not always happen. The general property of the decreasing returns case is that it gives rise to an upward- curving LTC curve and upward-sloping LAC and LMC curves. Note yet another application of the averagemarginal relationship: the fact that LMC exceeds LAC ensures that LAC must rise with output