[填空题]
The following diagram shows ic1uhc +eluf4(b 4wm4 the curve y=a sin dxl0j//dat f1 (k(x-d))+c where a, k, d and c are all positive constants. The curve haa djfl/d/t01 xs a minimum point at (1.5,1) and a maximum point at (3,7) .
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 .