Reasons why each one of those don't belong

1) 20+21 =41. Since 20^2+21^2 both have ^2 on top. The ^2 would stay the same. So that means the answer would be 41^2 for the first one which is why it is incorrect

2)Same for this. 4^2+5^2= 9^2

3)a^2+b^2 = ab^2 because since a and b are different letters/variables you cannot combine them therefore it is just ab.

4) (-1)^2 = 1 because -1 multiply by -1 again you would get positive 1. Next you do 1+0^2 = 1. Because 0^2 is just 0 because 0times 0 =0 so 1+0 = 1

Dbc if u do the math and stuff that what i got let me know if i was wrong