# Orthogonal projection, orthonormal basis, coordinate vector of the polynomial?

#### belleamie

Hey there I'm working on questions for a sample review for finals I'm stuck on these three I think I'm starting to confuse all the different theorem, I'm so lost please help

1) Find the coordinate vector of the polynomial
p(x)=1+x+x^2

relative to the following basis of P2:
p1=1+x, p2=1-x, p3=1+2x+3x^2

?????
I wasnt sure how to work this problem out:
Does it start out as?
b1=1,t,t^2
b2=t,1,t^2
b3= 1+t, 1-t, t-t^2

2) Let X be the linear span of the vectors
(1,1,1,1) (1,1,1,0) (1,1,0,0)
in R^4. Find the orthonormal basis for X?

It is:
[[u1]]^2
[[u2]]^2
[[u3]]^2

u1=1/2(1,1,1,1)
u2=1/6(1,1,1,0)
u3=1/4(1,1,0,0)

3) Let X be the linear span of the vectors
(1,2,1,2) (1,2,1,0) (1,1,0,0)
in R^4. Find the orthogonal projection of the vector (1,1,1,1) on th esubspace X?
It is solved like this:
c1=(v,u1)/(u1/u2)=(1+2+1+2)/(1+4+1+4)
c2=(v,u2)/(u2/u2)=(1+2+1+0)/(1+4+1+0)
c3=(v,u3)/(u3/u3)=(1+1+0+0)/(1+1+0+0)

there for x=proj(v,x) = c1u1+c2u2+c3u3

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#### shmoe

Homework Helper
belleamie said:
1) Find the coordinate vector of the polynomial
p(x)=1+x+x^2

relative to the following basis of P2:
p1=1+x, p2=1-x, p3=1+2x+3x^2
You want to find the vector [a,b,c] where p(x)=a*p1(x)+b*p2(x)+c*p3(x). This is the coordinate vector of p(x) with respect to your basis. I don't understand what followed, with the b's.

belleamie said:
2) Let X be the linear span of the vectors
(1,1,1,1) (1,1,1,0) (1,1,0,0)
in R^4. Find the orthonormal basis for X?

It is:
[[u1]]^2
[[u2]]^2
[[u3]]^2

u1=1/2(1,1,1,1)
u2=1/6(1,1,1,0)
u3=1/4(1,1,0,0)
An orthonormal basis is made up of orthogonal unit vectors.
Do you know the Gram-Schmidt orthogonalization algorithm? Use it to find an orthogonal basis, then make them unit vectors by dividing by their norms. Alternatively, you might be able to find an orthogonal basis by staring long enough, but Gram-Schmidt will work.

belleamie said:
3) Let X be the linear span of the vectors
(1,2,1,2) (1,2,1,0) (1,1,0,0)
in R^4. Find the orthogonal projection of the vector (1,1,1,1) on th esubspace X?
It is solved like this:
c1=(v,u1)/(u1/u2)=(1+2+1+2)/(1+4+1+4)
c2=(v,u2)/(u2/u2)=(1+2+1+0)/(1+4+1+0)
c3=(v,u3)/(u3/u3)=(1+1+0+0)/(1+1+0+0)

there for x=proj(v,x) = c1u1+c2u2+c3u3
In order to use that formula for the projection, you must use an orthogonal basis. Use Gram-Schmidt to get one.

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