Rút gọn

Let's simplify each expression one by one:

1) (91 - x^2)^2:
Expanding the expression, we get:
(91 - x^2)^2 = (91 - x^2)(91 - x^2)
= 91(91) - 91(x^2) - x^2(91) + x^2(x^2)
= 8281 - 91x^2 - 91x^2 + x^4
= 8281 - 182x^2 + x^4

2) 1 - x^2 + y^2:
This expression cannot be simplified further.

3) x / (x^2 + 1):
This expression cannot be simplified further.

4) (x^2 - xy + y^2) / (x^2 + xy + y^2):
This expression cannot be simplified further.

5) x^2/2 - y^3/3:
This expression cannot be simplified further.

6) -1/7:
This expression cannot be simplified further.

Therefore, the simplified expressions are:
1) (91 - x^2)^2 = 8281 - 182x^2 + x^4
2) 1 - x^2 + y^2
3) x / (x^2 + 1)
4) (x^2 - xy + y^2) / (x^2 + xy + y^2)
5) x^2/2 - y^3/3
6) -1/7