# Thread: Find the standard matrix of an orthogonal projection

1. ## Find the standard matrix of an orthogonal projection

Problem 4. Let W = Span[w1; w2], where
w1 = [3 0 4]
w2 = [4 5 -3]

If T (from R^3 to R^3) is the orthogonal projection onto W, find the standard matrix of T.

I did this by finding the orthogonal projections of e1, e2, and e3 on span W and constructing the transformation from [e1 e2 e3]

My answer was :
0.680 0.400 0.240
0.400 0.500 -0.300
0.240 -0.300 0.820

Is this correct? I double checked by finding that the applying the transformation matrix to w1 and w2 returned w1 and w2 which is what it should do. Thanks!

2. Originally Posted by eiad77
Problem 4. Let W = Span[w1; w2], where
w1 = [3 0 4]
w2 = [4 5 -3]

If T (from R^3 to R^3) is the orthogonal projection onto W, find the standard matrix of T.

I did this by finding the orthogonal projections of e1, e2, and e3 on span W and constructing the transformation from [e1 e2 e3]

My answer was :
0.680 0.400 0.240
0.400 0.500 -0.300
0.240 -0.300 0.820

Is this correct? I double checked by finding that the applying the transformation matrix to w1 and w2 returned w1 and w2 which is what it should do. Thanks!
Alternatively, you can use the fact that $\displaystyle w_1/5$ and $\displaystyle w_2/\sqrt{50}$ form an orthonormal basis for W to see that T is given by the formula $\displaystyle Tx = \tfrac1{25}\langle x,w_1\rangle w_1 + \tfrac1{50}\langle x,w_2\rangle w_2$. The resulting matrix agrees with the one above.

### find a projection p o f u onto span{w1, w2}

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