$2-18.$ The triangular plate ABC is deformed into the

shape shown by the dashed lines. If at $A,lo{n}_{AB}=0\mathrm{.}0075,$

$lo{n}_{AC}=0\mathrm{.}01$ and ${}_{xy}=0\mathrm{.}005$rad, determine the average

normal strain along edge $BC.$

Prob. $2-18$