It is known that the lengths of the side edges and the bottom edges of the triangular prism abc-a1b1c1 are equal, and the projection of A1 on the bottom ABC is the midpoint D of the BC edge, then the cosine value of the angle formed by the out of plane line AB and CC1 is: A、(√3)/4 B、(√5)/4 C、(√7)/4 D、3/4 Please give me a picture of this problem. There are a lot of pictures in the process of solution

It is known that the lengths of the side edges and the bottom edges of the triangular prism abc-a1b1c1 are equal, and the projection of A1 on the bottom ABC is the midpoint D of the BC edge, then the cosine value of the angle formed by the out of plane line AB and CC1 is: A、(√3)/4 B、(√5)/4 C、(√7)/4 D、3/4 Please give me a picture of this problem. There are a lot of pictures in the process of solution

Let the midpoint of BC be D, connecting a1d, ad and A1B, and it is easy to know that θ = ∠ a1ab is the angle between AB and CC1; and let the side edge and bottom side length of abc-a1b1c1 be 1, then | ad | = (√ 3) / 2 & nbsp;, | a1d | = & nbsp; 1 / 2, | A1B | = (√ 2) / 2 & nbsp; & nbsp;, cos θ = (& nbsp;) is obtained from cosine theorem