11. Use the digits 1-9 each once to form a two-digit perfect square, a three-digit perfect square, and a four-digit perfect square. What is the smallest four-digit perfect square among them? $\qquad$
【Answer】Solution:
$32 \times 32=1024$, contains 0, does not meet the requirement
$33 \times 33=1089$, contains 0, does not meet the requirement
$34 \times 34=1156$, contains two 1s, does not meet the requirement
$35 \times 35=1225$, contains two 2s, does not meet the requirement
$36 \times 36=1296$, the remaining digits $3, 4, 5, 7, 8$ do not form any two-digit perfect square, does not meet the requirement
$37 \times 37=1369$, the remaining digits $2, 4, 5, 7, 8$, where $25=5 \times 5, 784=28 \times 28$ meet the requirement, hence the answer is 1369