关系代数表达式π1，5，6（σ1>5(RXS)）怎么看懂？

数字说的是列号。
RXS 先用表R和S做笛卡尔积（知道怎么做吧，这个就不说了）
σ1>5 然后从RXS结果中选第1列大于第5列的元组
π1，5，6 最后取这些元组的第1，5，6列组成新表，即是最后结果

try to cpmare each tuple in R with S to find out wheather they equal in same attributes. If they equal, join them without repeated attribute (in this case, A B C should be the attribute of result.) and add them in the new table. The case you gave has no same tuple in R and S, so result is an empty table.
I think that you can find this in a db textbook, it's alway better to learn yourself than to ask.
Sorry there is no Chinese input here.

RXS：R和S是两张表，RXS是它们的笛卡尔积，我们假设结果为关系T
σ1>5(T)：选择T中第一列大于第五列的元组，我们假设为关系M
π1，5，6（M）：投影M中第一列、第五列和第六列，组成新的关系，也就是最终结构