
已知棱台\(ABC-A_{1}B_{1}C_{1}\),平面\(AA_{1}C_{1}C⊥\)平面\(A_{1}B_{1}C_{1}\),\(∠B_{1}A_{1}C_{1}=60°\),\(∠A_{1}B_{1}C_{1}=90°\),\(AA_{1}=AC=CC_{1}=\dfrac{A_{1}C_{1}}{2}\),\(D\),\(E\)分别是\(BC\)和\(A_{1}C_{1}\)的中点
\((I)\)证明:\(DE⊥B_{1}C_{1}\);
\((Ⅱ)\)求\(DE\)与平面\(BCC_{1}B_{1}\)所成角的余弦值