X = ∣ ∣ ∣ ∣ ∣ ∣ 0 3 1 4 2 8 4 4 1 1 ∣ ∣ ∣ ∣ ∣ ∣ + ∣ ∣ ∣ ∣ ∣ ∣ 0 − 3 1 4 5 4 4 0 7 ∣ ∣ ∣ ∣ ∣ ∣ + 4 ∣ ∣ ∣ ∣ ∣ ∣ 1 1 0 1 7 4 1 4 7 ∣ ∣ ∣ ∣ ∣ ∣
Find the value of X without expanding the determinants
(Using properties of determinants)
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Very nice ,short and sweet
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Thanks! Isnt this overrated?
Very good solutions. I didn't know the tricks.
very nice trick
Nice solution but you ignored my note that you can't expand the determinants you should convert the determinant that you get after adding them into triangular determinant then multiplying its elements in the main diagonal.
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Oops! I though that after adding them, I can expand it. I mean you just denied to expand in the beginning. Should I edit my solution? #Feeling Lazy
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To convert it into triangular form you can take common factor from R 1
4 ∣ ∣ ∣ ∣ ∣ ∣ 1 1 1 1 7 4 1 4 7 ∣ ∣ ∣ ∣ ∣ ∣
C 2 = C 1 × ( − 1 ) + C 2 C 3 = C 1 × ( − 1 ) + C 3
4 ∣ ∣ ∣ ∣ ∣ ∣ 1 1 1 0 6 3 0 3 6 ∣ ∣ ∣ ∣ ∣ ∣
C 3 = C 2 × ( 2 − 1 ) + C 3
4 ∣ ∣ ∣ ∣ ∣ ∣ 1 1 1 0 6 3 0 0 4 . 5 ∣ ∣ ∣ ∣ ∣ ∣
4 × [ 1 × 6 × 4 . 5 ] = 1 0 8
I find using row operations to reduce the final calculation is very effective. There are only two rules to it.
For 3 × 3 matrix, the calculations are much reduced if we create two zeros in a column. Let us look at the example provided by this problem.
X = ∣ ∣ ∣ ∣ ∣ ∣ 0 3 1 4 2 8 4 4 1 1 ∣ ∣ ∣ ∣ ∣ ∣ + ∣ ∣ ∣ ∣ ∣ ∣ 0 − 3 1 4 5 4 4 0 7 ∣ ∣ ∣ ∣ ∣ ∣ + 4 ∣ ∣ ∣ ∣ ∣ ∣ 1 1 0 1 7 4 1 4 7 ∣ ∣ ∣ ∣ ∣ ∣
= ∣ ∣ ∣ ∣ ∣ ∣ 0 0 1 4 − 2 2 8 4 − 2 9 1 1 ∣ ∣ ∣ ∣ ∣ ∣ + ∣ ∣ ∣ ∣ ∣ ∣ 0 0 1 4 1 7 4 4 2 1 7 ∣ ∣ ∣ ∣ ∣ ∣ + 4 ∣ ∣ ∣ ∣ ∣ ∣ 1 0 0 1 6 4 1 3 7 ∣ ∣ ∣ ∣ ∣ ∣
= ( 4 ) ( − 2 9 ) ( 1 ) − ( 4 ) ( − 2 2 ) ( 1 ) + ( 4 ) ( 2 1 ) ( 1 )
− ( 4 ) ( 1 7 ) ( 1 ) + 4 [ ( 1 ) ( 6 ) ( 7 ) − ( 1 ) ( 3 ) ( 4 ) ]
= − 1 1 6 − ( − 8 8 ) + 8 4 − 6 8 + 4 ( 4 2 − 1 2 )
= − 2 8 + 1 6 + 4 ( 3 0 ) = 1 0 8
Sir it must be 1 0 8
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Thanks. This old man have a lot of careless mistakes.
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No sir you are great , can you post a note of any math topic which you liked the most and explain it how you understood it , and share your experience , It would be great to see. Thanks.
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@U Z – I can't really think of anything to tell. I was trained an electrical engineer and not a mathematician. My second degree is an MBA. Later part of my career I was in corporate communication using my English language skills. I will think of something that is all I can promise.
X = ∣ ∣ ∣ ∣ ∣ ∣ 0 3 1 4 2 8 4 4 1 1 ∣ ∣ ∣ ∣ ∣ ∣ + ∣ ∣ ∣ ∣ ∣ ∣ 0 − 3 1 4 5 4 4 0 7 ∣ ∣ ∣ ∣ ∣ ∣ + 4 ∣ ∣ ∣ ∣ ∣ ∣ 1 1 0 1 7 4 1 4 7 ∣ ∣ ∣ ∣ ∣ ∣
1 s t d e t e r m i n a n t − R 1 c h a n g e s t o R 2
X = − ∣ ∣ ∣ ∣ ∣ ∣ 3 0 1 2 4 8 4 4 1 1 ∣ ∣ ∣ ∣ ∣ ∣
R 3 → R 3 − 3 1 R 1
X = − ∣ ∣ ∣ ∣ ∣ ∣ ∣ 3 0 0 2 4 3 2 2 4 4 3 2 9 ∣ ∣ ∣ ∣ ∣ ∣ ∣
Thus triangular matrix , therefore determinant = multiplication of the diagonals =
− ( 3 × 4 × 7 / 3 ) = − 2 8
(I am not showing this , try yourself)
Similarly doing for second(as done for first one) we get elements in the main diagonal , thus determinant =
− ( − 3 × 4 × 4 / 3 = 1 6
For third ,
∣ ∣ ∣ ∣ ∣ ∣ 1 1 0 1 7 4 1 4 7 ∣ ∣ ∣ ∣ ∣ ∣
R 2 → R 2 − R 1
∣ ∣ ∣ ∣ ∣ ∣ 1 0 0 1 6 4 1 3 7 ∣ ∣ ∣ ∣ ∣ ∣
R 3 → R 3 − 6 4 × R 2
∣ ∣ ∣ ∣ ∣ ∣ 1 1 0 1 7 0 1 4 5 ∣ ∣ ∣ ∣ ∣ ∣
once again triangular matrix therefore determinant = 1 × 6 × 5 = 3 0
X = − 2 8 + 1 6 + 4 ( 3 0 ) = 1 0 8
Shouldn't it be R 3 → R 3 − 3 1 R 1 in the first transformation?
basket method is one of the easiest solution to this kind of prblem
Can you explain the method here?
wat is basket method can u explain
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∣ ∣ ∣ ∣ ∣ ∣ 0 3 1 4 2 8 4 4 1 1 ∣ ∣ ∣ ∣ ∣ ∣ + ∣ ∣ ∣ ∣ ∣ ∣ 0 − 3 1 4 5 4 4 0 7 ∣ ∣ ∣ ∣ ∣ ∣ + 4 ∣ ∣ ∣ ∣ ∣ ∣ 1 1 0 1 7 4 1 4 7 ∣ ∣ ∣ ∣ ∣ ∣
Apply R 3 → R 3 − R 1 on first determinant and multiply scalar 4 to first row of third determinant.
∣ ∣ ∣ ∣ ∣ ∣ 0 3 1 4 2 4 4 4 7 ∣ ∣ ∣ ∣ ∣ ∣ + ∣ ∣ ∣ ∣ ∣ ∣ 0 − 3 1 4 5 4 4 0 7 ∣ ∣ ∣ ∣ ∣ ∣ + ∣ ∣ ∣ ∣ ∣ ∣ 4 1 0 4 7 4 4 4 7 ∣ ∣ ∣ ∣ ∣ ∣
Note that Row 1 and Row 3 in first and second matrix are same, so we can add second row to add determinants.
∣ ∣ ∣ ∣ ∣ ∣ 0 0 1 4 7 4 4 4 7 ∣ ∣ ∣ ∣ ∣ ∣ + ∣ ∣ ∣ ∣ ∣ ∣ 4 1 0 4 7 4 4 4 7 ∣ ∣ ∣ ∣ ∣ ∣
Now, second and third columns are same, so we can add column one!
∣ ∣ ∣ ∣ ∣ ∣ 4 1 1 4 7 4 4 4 7 ∣ ∣ ∣ ∣ ∣ ∣
Solving this determinant, 4 ( 4 9 − 1 6 ) − 4 ( 7 − 4 ) + 4 ( 4 − 7 ) = 1 0 8
Edit: If we have to do it by triangular matrix and not expanding,
4 ∣ ∣ ∣ ∣ ∣ ∣ 1 1 1 1 7 4 1 4 7 ∣ ∣ ∣ ∣ ∣ ∣
C 2 = C 1 × ( − 1 ) + C 2 C 3 = C 1 × ( − 1 ) + C 3
4 ∣ ∣ ∣ ∣ ∣ ∣ 1 1 1 0 6 3 0 3 6 ∣ ∣ ∣ ∣ ∣ ∣
C 3 = C 2 × ( 2 − 1 ) + C 3
4 ∣ ∣ ∣ ∣ ∣ ∣ 1 1 1 0 6 3 0 0 4 . 5 ∣ ∣ ∣ ∣ ∣ ∣
4 × [ 1 × 6 × 4 . 5 ] = 1 0 8