[填空题]
The following diagram s-lzyspb pgxc)h3s a:3z tm1**hows s(92(5yw,oulzyf k(u rd(u f tthe curve y=a sin (k(x-d))+c where a, k, d and c are all positive constants. The curve has a minimum point at (1.5,1) and a maximolt uwu5y2,k9 fs( ru fy(dz((um 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 .
