What values of a and b is there diminishing technical rate

Question

Problem

Suppose that the production function is f(x₁, x₂) = b(x₁^a + x₂^a), where a and b are positive constants.

1. For what values of a and b is there a diminishing technical rate of substitution?
2. For what values of a and b does the production function exhibit decreasing returns to scale?