а = -0,29, b = -0,78:
$$\sqrt{a-b} = \sqrt{-0,29-(-0,78)} = \sqrt{-0,29+0,78} = \sqrt{0,49} = 0,7$$
a = $$\frac{1}{4}$$, b = -$$\frac{4}{9}$$:
$$\sqrt{a-b} = \sqrt{\frac{1}{4} - (-\frac{4}{9})} = \sqrt{\frac{1}{4} + \frac{4}{9}} = \sqrt{\frac{9+16}{36}} = \sqrt{\frac{25}{36}} = \frac{5}{6}$$