[填空题]
The following diagram shows the curve y=a cos (k(x-t olu/xo :)x2md))+c where 6hbv0hqaxrg)a ( +/ek a, k, d and c are all positive constants. The curve has a minimum point at (1.5,2)6e(vak +aqbh0/hr)x g 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 .
