# Roots of polynomial equations ( Substitution )

#### Erfan

##### New member
How do I reduce u^4 + 5u^3 + 6u^2 + 5u + 1 = 0 to v^2 + 5v + 4 = 0 by using v = u + 1/u ?

#### M R

##### Active member
How do I reduce u^4 + 5u^3 + 6u^2 + 5u + 1 = 0 to v^2 + 5v + 4 = 0 by using v = u + 1/u ?
Since $$\displaystyle u \ne 0$$ you may divide through by $$\displaystyle u^2$$.

What do you notice now?

#### caffeinemachine

##### Well-known member
MHB Math Scholar
How do I reduce u^4 + 5u^3 + 6u^2 + 5u + 1 = 0 to v^2 + 5v + 4 = 0 by using v = u + 1/u ?
You can do as M R suggested or you can put $v=u+1/u$ in $v^2+5v+4=0$. You should get $u^4 + 5u^3 + 6u^2 + 5u + 1 = 0$.

#### M R

##### Active member
You can use the same idea to solve $$\displaystyle x^6-6x^5+14x^4-18x^3+14x^2-6x+1=0$$, which I made specially for you.

#### M R

##### Active member
And a slightly nicer one $$\displaystyle x^6-9x^5+29x^4-42x^3+29x^2-9x+1=0$$.