For those who don't know part 5 (Determinants)

Algebra Level 5

X = 0 4 4 3 2 4 1 8 11 + 0 4 4 3 5 0 1 4 7 + 4 1 1 1 1 7 4 0 4 7 X=\left| \begin{matrix} 0 & 4 & 4 \\ 3 & 2 & 4 \\ 1 & 8 & 11 \end{matrix} \right| +\left| \begin{matrix} 0 & 4 & 4 \\ -3 & 5 & 0 \\ 1 & 4 & 7 \end{matrix} \right| +4\left| \begin{matrix} 1 & 1 & 1 \\ 1 & 7 & 4 \\ 0 & 4 & 7 \end{matrix} \right|

Find the value of X X without expanding the determinants

(Using properties of determinants)


The answer is 108.

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4 solutions

Pranjal Jain
Jan 1, 2015

0 4 4 3 2 4 1 8 11 + 0 4 4 3 5 0 1 4 7 + 4 1 1 1 1 7 4 0 4 7 \left|\begin{matrix} 0 & 4 & 4\\3 & 2 & 4\\1 & 8 & 11\end{matrix}\right|+\left|\begin{matrix} 0 & 4 & 4\\-3 & 5 & 0\\1 & 4 & 7\end{matrix} \right|+4\left|\begin{matrix} 1 & 1 & 1\\1 & 7 & 4\\0 & 4 & 7\end{matrix}\right|

Apply R 3 R 3 R 1 R_{3}\rightarrow R_{3}-R_{1} on first determinant and multiply scalar 4 4 to first row of third determinant.

0 4 4 3 2 4 1 4 7 + 0 4 4 3 5 0 1 4 7 + 4 4 4 1 7 4 0 4 7 \left|\begin{matrix} 0 & 4 & 4\\3 & 2 & 4\\1 & 4 & 7\end{matrix}\right|+\left|\begin{matrix} 0 & 4 & 4\\-3 & 5 & 0\\1 & 4 & 7\end{matrix} \right|+\left|\begin{matrix} 4 & 4 & 4\\1 & 7 & 4\\0 & 4 & 7\end{matrix}\right|

Note that Row 1 and Row 3 in first and second matrix are same, so we can add second row to add determinants.

0 4 4 0 7 4 1 4 7 + 4 4 4 1 7 4 0 4 7 \left|\begin{matrix} 0 & 4 & 4\\0 & 7 & 4\\1 & 4 & 7\end{matrix}\right|+\left|\begin{matrix} 4 & 4 & 4\\1 & 7 & 4\\0 & 4 & 7\end{matrix}\right|

Now, second and third columns are same, so we can add column one!

4 4 4 1 7 4 1 4 7 \left|\begin{matrix} 4 & 4 & 4\\1 & 7 & 4\\1 & 4 & 7\end{matrix}\right|

Solving this determinant, 4 ( 49 16 ) 4 ( 7 4 ) + 4 ( 4 7 ) = 108 4(49-16)-4(7-4)+4(4-7)=108

Edit: If we have to do it by triangular matrix and not expanding,

4 1 1 1 1 7 4 1 4 7 4\begin{vmatrix} 1 & 1 & 1 \\ 1 & 7 & 4 \\ 1 & 4 & 7 \end{vmatrix}

C 2 = C 1 × ( 1 ) + C 2 C 3 = C 1 × ( 1 ) + C 3 { C }_{ 2 }=C_{ 1 }\times (-1)+{ C }_{ 2 }\\ { C }_{ 3 }=C_{ 1 }\times (-1)+{ C }_{ 3 }

4 1 0 0 1 6 3 1 3 6 4\begin{vmatrix} 1 & 0 & 0 \\ 1 & 6 & 3 \\ 1 & 3 & 6 \end{vmatrix}

C 3 = C 2 × ( 1 2 ) + C 3 { C }_{ 3 }=C_{ 2 }\times (\frac { -1 }{ 2 } )+{ C }_{ 3 }

4 1 0 0 1 6 0 1 3 4.5 4\begin{vmatrix} 1 & 0 & 0 \\ 1 & 6 & 0 \\ 1 & 3 & 4.5 \end{vmatrix}

4 × [ 1 × 6 × 4.5 ] = 108 4\times [1\times 6\times 4.5]=\boxed { 108 }

Very nice ,short and sweet

U Z - 6 years, 5 months ago

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Thanks! Isnt this overrated?

Pranjal Jain - 6 years, 5 months ago

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Yes , why he kept , he can only tell

U Z - 6 years, 5 months ago

Very good solutions. I didn't know the tricks.

Chew-Seong Cheong - 6 years, 5 months ago

very nice trick

Ranjitha Gowda - 6 years, 5 months ago

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.

Abdulrahman El Shafei - 6 years, 5 months ago

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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

Pranjal Jain - 6 years, 5 months ago

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you can copy it

0 4 4 3 2 4 1 8 11 + 0 4 4 3 5 0 1 4 7 + 4 1 1 1 1 7 4 0 4 7 \left|\begin{matrix} 0 & 4 & 4\\3 & 2 & 4\\1 & 8 & 11\end{matrix}\right|+\left|\begin{matrix} 0 & 4 & 4\\-3 & 5 & 0\\1 & 4 & 7\end{matrix} \right|+4\left|\begin{matrix} 1 & 1 & 1\\1 & 7 & 4\\0 & 4 & 7\end{matrix}\right| Apply R 3 R 3 R 1 R_{3}\rightarrow R_{3}-R_{1} on first determinant and multiply scalar 4 4 to first row of third determinant. 0 4 4 3 2 4 1 4 7 + 0 4 4 3 5 0 1 4 7 + 4 4 4 1 7 4 0 4 7 \left|\begin{matrix} 0 & 4 & 4\\3 & 2 & 4\\1 & 4 & 7\end{matrix}\right|+\left|\begin{matrix} 0 & 4 & 4\\-3 & 5 & 0\\1 & 4 & 7\end{matrix} \right|+\left|\begin{matrix} 4 & 4 & 4\\1 & 7 & 4\\0 & 4 & 7\end{matrix}\right| Note that Row 1 and Row 3 in first and second matrix are same, so we can add second row to add determinants. 0 4 4 0 7 4 1 4 7 + 4 4 4 1 7 4 0 4 7 \left|\begin{matrix} 0 & 4 & 4\\0 & 7 & 4\\1 & 4 & 7\end{matrix}\right|+\left|\begin{matrix} 4 & 4 & 4\\1 & 7 & 4\\0 & 4 & 7\end{matrix}\right| Now, second and third columns are same, so we can add column one! 4 4 4 1 7 4 1 4 7 \left|\begin{matrix} 4 & 4 & 4\\1 & 7 & 4\\1 & 4 & 7\end{matrix}\right|

To convert it into triangular form you can take common factor from R 1 R_{1}

4 1 1 1 1 7 4 1 4 7 4\begin{vmatrix} 1 & 1 & 1 \\ 1 & 7 & 4 \\ 1 & 4 & 7 \end{vmatrix}

C 2 = C 1 × ( 1 ) + C 2 C 3 = C 1 × ( 1 ) + C 3 { C }_{ 2 }=C_{ 1 }\times (-1)+{ C }_{ 2 }\\ { C }_{ 3 }=C_{ 1 }\times (-1)+{ C }_{ 3 }

4 1 0 0 1 6 3 1 3 6 4\begin{vmatrix} 1 & 0 & 0 \\ 1 & 6 & 3 \\ 1 & 3 & 6 \end{vmatrix}

C 3 = C 2 × ( 1 2 ) + C 3 { C }_{ 3 }=C_{ 2 }\times (\frac { -1 }{ 2 } )+{ C }_{ 3 }

4 1 0 0 1 6 0 1 3 4.5 4\begin{vmatrix} 1 & 0 & 0 \\ 1 & 6 & 0 \\ 1 & 3 & 4.5 \end{vmatrix}

4 × [ 1 × 6 × 4.5 ] = 108 4\times [1\times 6\times 4.5]=\boxed { 108 }

Abdulrahman El Shafei - 6 years, 5 months ago

I find using row operations to reduce the final calculation is very effective. There are only two rules to it.

  1. The value of determinant does not change if we replace a r o w i row_i with r o w i C ˙ r o w j row_i - C \dot{}row_j .
  2. When two rows are swapped the resultant determinant is multiplied by 1 -1 .

For 3 × 3 3\times 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 4 4 3 2 4 1 8 11 + 0 4 4 3 5 0 1 4 7 + 4 1 1 1 1 7 4 0 4 7 X = \begin{vmatrix} 0&4&4 \\ 3&2&4 \\ 1&8&11 \end{vmatrix} + \begin{vmatrix} 0&4&4 \\ -3&5&0 \\ 1&4&7 \end{vmatrix} + 4 \begin{vmatrix} 1&1&1\\ 1&7&4\\ 0&4&7 \end{vmatrix}

= 0 4 4 0 22 29 1 8 11 + 0 4 4 0 17 21 1 4 7 + 4 1 1 1 0 6 3 0 4 7 \quad = \begin{vmatrix} 0&4&4 \\ 0&-22&-29 \\ 1&8&11 \end{vmatrix} + \begin{vmatrix} 0&4&4 \\ 0&17&21 \\ 1&4&7 \end{vmatrix} + 4 \begin{vmatrix} 1&1&1\\ 0&6&3\\ 0&4&7 \end{vmatrix}

= ( 4 ) ( 29 ) ( 1 ) ( 4 ) ( 22 ) ( 1 ) + ( 4 ) ( 21 ) ( 1 ) \quad = (4)(-29)(1)-(4)(-22)(1)+(4)(21)(1)

( 4 ) ( 17 ) ( 1 ) + 4 [ ( 1 ) ( 6 ) ( 7 ) ( 1 ) ( 3 ) ( 4 ) ] \quad \quad -(4)(17)(1)+4[(1)(6)(7)-(1)(3)(4)]

= 116 ( 88 ) + 84 68 + 4 ( 42 12 ) \quad = -116- (-88)+84-68+4(42-12)

= 28 + 16 + 4 ( 30 ) = 108 \quad =-28+16+4(30) = \boxed{108}

Sir it must be 108 \boxed{108}

Parth Lohomi - 6 years, 5 months ago

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Thanks. This old man have a lot of careless mistakes.

Chew-Seong Cheong - 6 years, 5 months ago

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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.

U Z - 6 years, 5 months ago

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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.

Chew-Seong Cheong - 6 years, 5 months ago
U Z
Jan 1, 2015

X = 0 4 4 3 2 4 1 8 11 + 0 4 4 3 5 0 1 4 7 + 4 1 1 1 1 7 4 0 4 7 X=\left| \begin{matrix} 0 & 4 & 4 \\ 3 & 2 & 4 \\ 1 & 8 & 11 \end{matrix} \right| +\left| \begin{matrix} 0 & 4 & 4 \\ -3 & 5 & 0 \\ 1 & 4 & 7 \end{matrix} \right| +4\left| \begin{matrix} 1 & 1 & 1 \\ 1 & 7 & 4 \\ 0 & 4 & 7 \end{matrix} \right|


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 1^{st} determinant - R_{1} ~changes~ to R_{2}

X = 3 2 4 0 4 4 1 8 11 X= - \left| \begin{matrix} 3 & 2 & 4 \\ 0 & 4 & 4 \\ 1 & 8 & 11 \end{matrix} \right|

R 3 R 3 1 3 R 1 R_{3} \to R_{3} - \dfrac{1}{3}R_{1}

X = 3 2 4 0 4 4 0 22 3 29 3 X= - \left| \begin{matrix} 3 & 2 & 4 \\ 0 & 4 & 4 \\ 0 & \dfrac{22}{3} & \dfrac{29}{3} \end{matrix} \right|

Thus triangular matrix , therefore determinant = multiplication of the diagonals =

( 3 × 4 × 7 / 3 ) = 28 - (3\times4\times7/3) = -28


(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 = 16 - (-3\times 4 \times 4/3 = 16


For third ,

1 1 1 1 7 4 0 4 7 \left|\begin{matrix} 1 & 1 & 1 \\ 1 & 7 & 4 \\ 0 & 4 & 7 \end{matrix} \right|

R 2 R 2 R 1 R_{2} \to R_{2} - R_{1}

1 1 1 0 6 3 0 4 7 \left|\begin{matrix} 1 & 1 & 1 \\ 0 & 6 & 3 \\ 0 & 4 & 7 \end{matrix} \right|

R 3 R 3 4 6 × R 2 R_{3} \to R_{3} - \dfrac{4}{6} \times R_{2}

1 1 1 1 7 4 0 0 5 \left|\begin{matrix} 1 & 1 & 1 \\ 1 & 7 & 4 \\ 0 & 0 & 5 \end{matrix} \right|

once again triangular matrix therefore determinant = 1 × 6 × 5 = 30 1\times6\times5 = 30

X = 28 + 16 + 4 ( 30 ) = 108 X = -28 + 16 + 4(30) = 108

Shouldn't it be R 3 R 3 1 3 R 1 R_{3}\rightarrow R_{3}-\frac{1}{3} R_{1} in the first transformation?

Pranjal Jain - 6 years, 5 months ago

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Yes sorry ......

U Z - 6 years, 5 months ago
Roland Casuga
Dec 31, 2014

basket method is one of the easiest solution to this kind of prblem

Can you explain the method here?

Pranjal Jain - 6 years, 5 months ago

wat is basket method can u explain

prajwal kavad - 6 years, 5 months ago

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