$$b = \pi(a)\;\; \implies\;\; b \;= \;10\pi\pm 1\cdot \pi \;= \;10\pi \pm \pi$$
$$\implies\;\; \delta b \;\;= \;\;\pi \;\;= \;\;\pi(\delta a)$$
$$b = \pi(a)\;\; \implies\;\; b \;= \;10\pi\pm 1\cdot \pi \;= \;10\pi \pm \pi$$
$$\implies\;\; \delta b \;\;= \;\;\pi \;\;= \;\;\pi(\delta a)$$