# Find the value of the sum in the expression

#### Amelia

##### New Member
Consider the matrix below. Calculate the expression using Excel. What is the value of the sum of the diagonal elements? a11 + a22.

Could somebody help me with this question please?

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#### Amelia

##### New Member
Thanks for the answer. I did the calculation using the function: =SUM(MMULT({Matrix_1};{Matrix_2})) and resulted in the value 4.
Can anyone tell me if this is wrong?

##### MrExcel MVP
Thanks for the answer. I did the calculation using the function: =SUM(MMULT({Matrix_1};{Matrix_2})) and resulted in the value 4.
Can anyone tell me if this is wrong?

This looks like home work. Try to calculate what you need first manually. How the calculation is done is described in any introduction to linear algebra, showing calculations with matrices.

#### Amelia

##### New Member
No, it's not. I don't have computer class in high school. I'm just training a little of Excel with lists of exercises. But thanks for your help anyway.

##### MrExcel MVP
No, it's not. I don't have computer class in high school. I'm just training a little of Excel with lists of exercises. But thanks for your help anyway.

Good idea. What results are you expecting to see with those matrices? Are you multiplying or adding?

#### Amelia

##### New Member
Certainly I hope to see an addition, as in the end of this question says 'a11+a22'..

##### MrExcel MVP
Certainly I hope to see an addition, as in the end of this question says 'a11+a22'..

Still not very clear.

The following shows adding two matrices you have shown:

 7 4 0 -2 7 2 1 1 3 1 4 2

<COLGROUP><COL style="WIDTH: 48pt" span=8 width=64><TBODY>
</TBODY>

First matrix is located in A2:B3, the second in D2:E3, and the result matrix obtained by addition in G2:H3.

In G2 enter and copy down:

=A2+D2

In H2 enter and copy down:

=B2+E2

If you mean by adding 'a11+a22' the sum of G2 and H3 of the third matrix wit as result 9...

Control+shift+enter, not just enter:

=SUM(N(OFFSET(G2,ROW(INDIRECT("1:2"))-1,ROW(INDIRECT("1:2"))-1)))

which is derived from the link I quoted earlier.

#### Amelia

##### New Member
Now I understand your explanation. This is really complicated to understand, I mean what the question asks.
This range of cells in which the question says: 'a11+a22'... It was totally confused to me!
But I think your explanation is correct. You have very good logical reasoning.
Thanks, you helped me a lot, really.

Last edited:

##### MrExcel MVP
Now I understand your explanation. This is really complicated to understand, I mean what the question asks.
This range of cells in which the question says: 'a11+a22'... It was totally confused to me!
But I think your explanation is correct. You have very good logical reasoning.
Thanks, you helped me a lot, really.

You are welcome.

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