[填空题]
The following diagram shows the curve y=a cos (k(x-x gceje,li qs 69s;j0qym5 +ehq -t:a*d))+c where r s;vonj)xq9lb 3v;5w a, k, d and c are all pos;bxj nov)l ws9q35v;ritive constants. The curve has a minimum point at (1.5,2) and a maximum point at (3.5,7) .
1. Write down the value of a and the value of c . a = c =
2. Find the value of k .
3. Find the smallest possible value of d , given d>0 .
