matlab solver with respect to x^2

I have some questions for matlab solve

If I have an equation like x^2+x^4=a and try to find solution, I could use matlab solve

My question is instead to solve x, is there any way to solve equation with respect to x^2...?

For example

equ1=x^2+x^4==a

S=solve(equ1,x^2)....?

another question is if I want to solve the above equation with a=1

How to use "solve"(S=solve(equ1, x,a==1) is not working properly...

Answer 1

[Chunru]

syms x a y
equ1=x^2+x^4==a 

equ1 = 

equ2 = subs(equ1, x^2, y)

equ2 = 

S=solve(equ1,x)

S = 

Sx2 =solve(equ2)

Sx2 = 

subs(Sx2, a, 1)

ans = 

[Jinho Lee]

Thanks a lot!