18. 如图,$AB// CD$,BF,DF分别平分$∠ABE$和$∠CDE,BF// DE,∠F$与$∠ABE$互补,则$∠F$的度数为____.

$108^{\circ}$

19. (8分)计算:
(1)$\sqrt {2}+3\sqrt {2}-5\sqrt {2}$;
(2)$\sqrt [3]{8}+\sqrt {(-2)^{2}}-\sqrt {\frac {1}{4}}$;
(3)$(-2)^{3}×\sqrt {(-4)^{2}}+\sqrt [3]{(-4)^{3}}×(\frac {1}{2})^{2}-\sqrt {9}$;
(4)$\sqrt [3]{-27}-\sqrt {0}-\sqrt {\frac {1}{4}}+\sqrt [3]{0.125}+\sqrt [3]{1-\frac {63}{64}}$.

(1)$-\sqrt {2}$；(2)$\frac{7}{2}$；(3)$-36$；(4)$-\frac{11}{4}$