(a)

Find the sign of .

The second partial derivative determines the nature of concavity of the function.

Observe the graph:

The function is concave up as it moves along positive -axis at the point .

Hence, is positive.

(b)

Find the sign of .

Observe the graph:

The function is concave down as it moves along negative -axis at the point .

Hence, is negative.

(a) is positive.

(b) is negative.