6.
\((1)\)化简\((\overrightarrow{AB}-\overrightarrow{CD})+(\overrightarrow{BE}-\overrightarrow{DE})\)______ .
\((2)\)若\(\overrightarrow{a}{=}(2{,}3){,}\overrightarrow{b}{=}({-}4{,}7)\),则\(\overrightarrow{a}\)在\(\overrightarrow{b}\)上的投影为__ ____ 。
\((3)\)已知\(\sin 110^{{∘}}{=}a\),则\(\cos 20^{{∘}}\)等于__________。
\((4)\)下列说法:
\({①}\)正切函数\(y{=}\tan x\)在定义域内是增函数;
\({②}\)函数\(f(x){=}\cos(\dfrac{2}{3}x{+}\dfrac{\pi}{2})\)是奇函数;
\({③}x{=}\dfrac{\pi}{8}\)是函数\(y{=}\sin(2x{+}\dfrac{5}{4}\pi)\)的一条对称轴方程;
其中正确的是______\({.}(\)写出所有正确答案的序号\()\)