[填空题]
The following diagram shows the curve y(;kskld6 /(gezgu(+cv .bpl j=a cos (k(x-d))+cuar(ak ;oy j8bw46 3sz where a, k, d and c are al6abu3k ;8jwyz(4 aosrl positive 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 .
