# HL Matrix Arithmetic, Inverse & Sim Equations 1.14

## What are matrices and how do we calculate with them?

On this page you can find a set of videos, created by James Tanton (training available here), for the topic AHL 1.14 that provide an intuitive, conceptual insight into what matrices are, why they are useful and how we calculate with them. Calculating a matrix's inverse and solving simultaneous equations using matrices are also covered [note: the film 'The Matrix' (pause the clip at 34, 58, 66/67, 68/69 and 80 seconds to see 'the matrix'!) was not created by James, nor us! Tok: what's the link between the film and this topics?]

A practice question, 68 marks of IB Exam Style questions and a bonus section on Associativity of Matrices follow at the end of the videos.

## Matrices 6: Formal Associativity of Matrices

Associative: re-arranging which items you bracket won't change the outcome, as with equations that only contain multiplication or only addition, but not in a calculation with a mix of multiplications and additions.

Mutiplication and addition do not, combined, show associativity e.g. (1+1)x2 ≠ 1+(1x2) [not associative] but (3x4)x5 = 3x(4x5) [associative].

## Practice question

Simultaneous equation question

Q1) Solve the following (unfriendly) system of equations using matrix methods.

$$\begin{array}{l}3x + 1.7y = e\\3x - 3.3y = \pi \end{array} % MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGceaqabeaacaaIZa % GaamiEaiabgUcaRiaaigdacaGGUaGaaG4naiaadMhacqGH9aqpcaWG % LbaabaGaaG4maiaadIhacqGHsislcaaIZaGaaiOlaiaaiodacaWG5b % Gaeyypa0JaeqiWdahaaaa!4647!$$

SOLUTION

Set the matrix: $$A = \left( {\begin{array}{*{20}{c}}3&{1.7}\\3&{ - 3.3}\end{array}} \right) % MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyqaiabg2 % da9maabmaabaqbaeqabiGaaaqaaiaaiodaaeaacaaIXaGaaiOlaiaa % iEdaaeaacaaIZaaabaGaeyOeI0IaaG4maiaac6cacaaIZaaaaaGaay % jkaiaawMcaaaaa!401A!$$

Then the system of equations reads:

$$A\left( {\begin{array}{*{20}{c}}x\\y\end{array}} \right) = \left( {\begin{array}{*{20}{c}}e\\\pi \end{array}} \right) % MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyqamaabm % aabaqbaeqabiqaaaqaaiaadIhaaeaacaWG5baaaaGaayjkaiaawMca % aiabg2da9maabmaabaqbaeqabiqaaaqaaiaadwgaaeaacqaHapaCaa % aacaGLOaGaayzkaaaaaa!3F8E!$$

The matrix  is invertible and so the system has the unique solution:

$$\left( {\begin{array}{*{20}{c}}x\\y\end{array}} \right) = {A^{ - 1}}\left( {\begin{array}{*{20}{c}}e\\\pi \end{array}} \right) = - \frac{1}{{15}}\left( {\begin{array}{*{20}{c}}{ - 3.3}&{ - 1.7}\\{ - 3}&3\end{array}} \right)\left( {\begin{array}{*{20}{c}}e\\\pi \end{array}} \right) % MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaeWaaeaafa % qabeGabaaabaGaamiEaaqaaiaadMhaaaaacaGLOaGaayzkaaGaeyyp % a0JaamyqamaaCaaaleqabaGaeyOeI0IaaGymaaaakmaabmaabaqbae % qabiqaaaqaaiaadwgaaeaacqaHapaCaaaacaGLOaGaayzkaaGaeyyp % a0JaeyOeI0YaaSaaaeaacaaIXaaabaGaaGymaiaaiwdaaaWaaeWaae % aafaqabeGacaaabaGaeyOeI0IaaG4maiaac6cacaaIZaaabaGaeyOe % I0IaaGymaiaac6cacaaI3aaabaGaeyOeI0IaaG4maaqaaiaaiodaaa % aacaGLOaGaayzkaaWaaeWaaeaafaqabeGabaaabaGaamyzaaqaaiab % ec8aWbaaaiaawIcacaGLPaaaaaa!5416!$$

which gives:

$$\begin{array}{l}x = \frac{1}{{15}}\left( {3.3e + 1.7\pi } \right)\\y = \frac{1}{5}\left( {e - \pi } \right)\end{array} % MathType!MTEF!2!1!+- % feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGceaqabeaacaWG4b % Gaeyypa0ZaaSaaaeaacaaIXaaabaGaaGymaiaaiwdaaaWaaeWaaeaa % caaIZaGaaiOlaiaaiodacaWGLbGaey4kaSIaaGymaiaac6cacaaI3a % GaeqiWdahacaGLOaGaayzkaaaabaGaamyEaiabg2da9maalaaabaGa % aGymaaqaaiaaiwdaaaWaaeWaaeaacaWGLbGaeyOeI0IaeqiWdahaca % GLOaGaayzkaaaaaaa!4C5A!$$

## IB exam style questions

Matrix Arithemetic, Determinant, Inverse and simultaneous equations :- use these Exam style questions to test if you feel ready, or what you need to review, ahead of moving on to the new topic or your end of unit test/mocks/final exams!

SOLUTIONS to Exam style questions

## Bonus Material: Matrix Multiplication is Associative

Associative: re-arranging which items you bracket won't change the outcome, as with equations that only contain multiplication or only addition, but not in a calculation with a mix of multiplications and additions.

Mutiplication and addition do not, combined, show associativity e.g. (1+1)x2 ≠ 1+(1x2) [not associative] but (3x4)x5 = 3x(4x5) [associative].

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