Not easy:
- 4 < x^4 + 4x^2 ---> 0 < x^4+4x^2+4 = (x^2+2)^2, so it is always positive. Always satisfied. Curious ...
Now the other one:
x^4+4x^2<21 --> x^4+4x^2-21 < 0 --> (x^2+2)^2 - 25 < 0
(x^2+2)^2<25 ->>> x^2+2 < 5 ---> x^2 <3,
So -sqrt(3)<x<sqrt(3) is the answer! Brainliest?
