Here, if we let *u *=* x*^{2}*,* we can rewrite the equation so it looks like an ordinary quadratic equation:

u^{2}− 20u+ 64 = 0

We now factor to give:

(

u− 16)(u− 4) = 0

So the solutions for *u* are 16 or 4.

So `x^2= 16` or `x^2= 4`.

These give us:

x = −4 or 4 |
x = −2 or 2 |

So the complete set of solutions is: `x = −4, −2, 2, 4`.

Is it correct?

The sketch shows:

`y = x^4 - 20x^2 + 64`, showing intersections with the *x*-axis

We can see from where the graph cuts the *x*-axis that the solutions are correct.

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