Mathematics for Master Students
Which tasks gave you trouble? Bring them up now. We take the time to work through them before moving on.
Question
The roastery sells \(D(p) = 1200 - 20p\) bags a week at price \(p\) and can roast at most \(500\). Write “sell exactly 500” as two lines, \(q = 1200 - 20p\) and \(q = 500\). At which price do they cross?
The roastery’s house blend must hit a weight and a cost target at once: how many kg of each bean? Today: turn every target into an equation, then find the point that meets them all.
The roastery’s house blend mixes a base bean at €4/kg and a premium bean at €8/kg. This week’s batch must weigh 20 kg and cost €5 per kg on average, the €5 per (1 kg) bag in its cost \(C(q) = 200 + 5q\).
Question
How many kg of each bean?
With \(x\) kg base and \(y\) kg premium, the weight target is \(x + y = 20\) and the cost target is \(4x + 8y = 20 \cdot 5 = 100\) (€100 for 20 kg). Does the cost target on its own fix the blend?
One condition in two unknowns is a whole line: every dot costs €100, but the batch weighs 25 or 15 kg. The cost target alone cannot choose.
Question
Which blend costs exactly €100?
| (a) \(8\) kg base, \(9\) kg premium | (b) \(9\) kg base, \(8\) kg premium |
| (c) \(10\) kg base, \(10\) kg premium | (d) \(12\) kg base, \(5\) kg premium |
(b): \(4 \cdot 9 + 8 \cdot 8 = 36 + 64 = 100\) ✓. The others cost €104, €120 and €88.
But (b) weighs only \(17\) kg, and (c) weighs the right \(20\) kg at the wrong price. Each target alone has many answers; we need both at once.
Solve one equation for one variable, then substitute into the other.
Scale the equations so one variable cancels when you subtract.
Question
Two trucks carry \(10\) pallets in total, and the first carries \(4\) more than the second: \(x + y = 10, \quad x - y = 4\). Ideas for \((x, y)\)?
Adding instead of subtracting is elimination too: whichever move makes a variable cancel.
Every condition today drew a straight line. Which equations do that? An equation is linear when every unknown appears on its own, to the first power, nothing more exotic.
Question
Is \(\frac{x}{2} = 7 - 3y\) a linear equation in \(x\) and \(y\)?
At which price is there neither a queue nor unsold stock?
From here on \(D(p)\) is a whole market’s demand, not the roastery’s:
Look where the lines cross: about €30.
Question
The price is set at €40, above the equilibrium of about €30. Do sellers offer more than buyers want (a surplus, excess supply) or less (a shortage, excess demand)?
The picture says about €30. Elimination makes it exact: move the \(p\)-terms of \(q = 120 - 2p\) and \(q = 3p - 30\) to the left: \[q + 2p = 120, \qquad q - 3p = -30.\]
Question
Another market: demand \(D(p) = 100 - p\), supply \(S(p) = 2p - 20\). What is the equilibrium price?
\[100 - p = 2p - 20 \quad \Rightarrow \quad 120 = 3p \quad \Rightarrow \quad p = 40\]
Check the supply side: \(2 \cdot 40 - 20 = 60\) ✓. Price €40, quantity \(q = 100 - 40 = 60\) units.
The workshop makes \(x\), \(y\) and \(z\) units of three products this week:
| per unit | \(x\) | \(y\) | \(z\) | available |
|---|---|---|---|---|
| Packaging (units) | 1 | 1 | 1 | 9 |
| Labour (h) | 1 | 2 | 2 | 16 |
| Machine (h) | 2 | 1 | 3 | 17 |
Every resource must be used up, so each row is one condition: \(x + y + z = 9\), \(x + 2y + 2z = 16\), \(2x + y + 3z = 17\).
Question
Method 2 turned two unknowns into one. What would you do with three?
Question
A system arrives in this shape: \(x + 2y + z = 10, \quad y - z = 1, \quad z = 2\). What is \(x\)?
Solution \((2, 3, 2)\). Each equation has one unknown fewer than the one above: a triangular form, solved from the bottom up.
Eq. 1: \(x + y + z = 9\)
Eq. 2: \(x + 2y + 2z = 16\)
Eq. 3: \(2x + y + 3z = 17\)
Method 2 again, twice: subtract multiples of the first equation so that \(x\) cancels.
Equal minus equal stays equal, so no solution is lost. Adding Eq. 1 back undoes the step, so no new one appears.
We copied \(x\), \(y\), \(z\) and every plus sign again and again, yet only the numbers changed. Keep just the numbers, the augmented matrix (\(R_1, R_2, R_3\) name the rows):
\[\left[\begin{array}{ccc|c} 1 & 1 & 1 & 9 \\ 1 & 2 & 2 & 16 \\ 2 & 1 & 3 & 17 \end{array}\right] \;\xrightarrow[\;R_3 \,\to\, R_3 - 2R_1\;]{\;R_2 \,\to\, R_2 - R_1\;}\; \left[\begin{array}{ccc|c} 1 & 1 & 1 & 9 \\ 0 & 1 & 1 & 7 \\ 0 & -1 & 1 & -1 \end{array}\right]\]
Each row is an equation and the bar stands for the equals sign. The step we just did by hand is now one arrow. No matrix algebra, pure bookkeeping.
Question
Rows \(R_1 = (1, 2, -1 \,|\, 3)\) and \(R_2 = (2, 5, 1 \,|\, 9)\). Compute \(R_2 - 2R_1\).
\[\begin{aligned}(2, 5, 1 \,|\, 9) - 2 \cdot (1, 2, -1 \,|\, 3) &= (2 - 2,\; 5 - 4,\; 1 + 2 \,|\, 9 - 6) \\ &= (0, 1, 3 \,|\, 3)\end{aligned}\]
Common Mistake
\(1 - 2 \cdot (-1) = 1 + 2 = 3\), not \(-1\): subtracting a negative adds.
Whatever is fair for equations is fair for rows. Three row operations:
One test covers all three: can you undo it? Swap back, divide back, subtract back. A move you can undo neither loses nor creates a solution: the solution set stays the same.
Question
Which of these is not an allowed row operation?
| (a) Swap \(R_1\) and \(R_3\) | (b) Replace \(R_2\) by \(R_2 - 3R_1\) |
| (c) Multiply \(R_3\) by \(0\) | (d) Divide \(R_2\) by \(2\) |
(c): multiplying by \(0\) turns the row into \(0 = 0\), and nothing multiplies it back. The condition is gone for good, and the solution set can grow.
Swap, divide by \(2\) and subtract \(3R_1\) can all be undone, so the solution set stays the same.
Rows 2 and 3 read \(y + z = 7\) and \(-y + z = -1\), the Two Trucks shape. Add them, then divide by 2:
\[\left[\begin{array}{ccc|c} 1 & 1 & 1 & 9 \\ 0 & 1 & 1 & 7 \\ 0 & -1 & 1 & -1 \end{array}\right] \;\xrightarrow[\;R_3 \,\to\, R_3 / 2\;]{\;R_3 \,\to\, R_3 + R_2\;}\; \left[\begin{array}{ccc|c} 1 & 1 & 1 & 9 \\ 0 & 1 & 1 & 7 \\ 0 & 0 & 1 & 3 \end{array}\right]\]
The workshop’s system is now in triangular form, like the one from the start of this section. Read it from the bottom up:
\[x + y + z = 9, \qquad y + z = 7, \qquad z = 3\]
Plug the triple \((2, 4, 3)\) back into the original three equations:
Make \(2\) of \(x\), \(4\) of \(y\), \(3\) of \(z\): every resource used to the last unit.
Common Mistake
When you apply a row operation, act on the whole row, the right-hand side included. Subtracting \(2R_1\) from \(R_3\) but forgetting to also do \(17 - 2 \cdot 9 = -1\) silently corrupts the whole solve.
Question
A colleague proposes the plan \((x, y, z) = (3, 3, 3)\) instead. Which resource does it exceed?
| (a) Packaging (\(9\)) | (b) Labour (\(16\) h) |
| (c) Machine (\(17\) h) | (d) None, it also works |
(c), with \((3, 3, 3)\):
| resource | used | available | verdict |
|---|---|---|---|
| Packaging | \(3 + 3 + 3 = 9\) | \(9\) | ✓ |
| Labour | \(3 + 6 + 6 = 15\) | \(16\) | 1 h idle |
| Machine | \(6 + 3 + 9 = 18\) | \(17\) | exceeded |
A linear system with a unique solution has exactly one plan that uses every resource fully. Any other plan breaks a limit, leaves something idle, or, like \((3, 3, 3)\), both.
Work on one column at a time and write every intermediate matrix down. Elimination fails through slips, not through difficulty.
Question
Two straight lines on a page. How many points can they share? Could it be exactly two?
\(x + y = 4\) and \(x + y = 6\) are parallel, same slope, different heights. They never meet, so no pair \((x, y)\) satisfies both.
\(2x + 2y = 8\) is just \(x + y = 4\) doubled, the same line. Every point on it solves both; there are infinitely many solutions.
When one equation adds nothing new, we describe all solutions with a free parameter.
For \(x + y = 4\) with a redundant partner:
Question
The solution set is \((x, y) = (4 - t,\; t)\). Which point belongs to it?
| (a) \((5, 1)\) | (b) \((1, 3)\) |
| (c) \((3, 2)\) | (d) \((2, 4)\) |
(b): \(t = 3\) gives \((4 - 3, 3) = (1, 3)\). Equivalently, \(1 + 3 = 4\) ✓.
The quick test is the original equation \(x + y = 4\): the other three sum to \(6\), \(5\) and \(6\).
With three unknowns, each equation is a plane in space, but the same three fates return:
The workshop replaces its machine. The new one needs \(2, 3, 3\) hours per product, and \(20\) hours are available: \(2x + 3y + 3z = 20\)
\[\left[\begin{array}{ccc|c} 1 & 1 & 1 & 9 \\ 1 & 2 & 2 & 16 \\ 2 & 3 & 3 & 20 \end{array}\right] \;\xrightarrow[\;R_3 \,\to\, R_3 - 2R_1\;]{\;R_2 \,\to\, R_2 - R_1\;}\; \left[\begin{array}{ccc|c} 1 & 1 & 1 & 9 \\ 0 & 1 & 1 & 7 \\ 0 & 1 & 1 & 2 \end{array}\right]\]
\[\left[\begin{array}{ccc|c} 1 & 1 & 1 & 9 \\ 0 & 1 & 1 & 7 \\ 0 & 1 & 1 & 2 \end{array}\right] \;\xrightarrow{\;R_3 \,\to\, R_3 - R_2\;}\; \left[\begin{array}{ccc|c} 1 & 1 & 1 & 9 \\ 0 & 1 & 1 & 7 \\ 0 & 0 & 0 & -5 \end{array}\right]\]
With \(25\) hours instead, the value that packaging and labour already imply:
\[\left[\begin{array}{ccc|c} 1 & 1 & 1 & 9 \\ 1 & 2 & 2 & 16 \\ 2 & 3 & 3 & 25 \end{array}\right] \;\xrightarrow[\;R_3 \,\to\, R_3 - 2R_1\;]{\;R_2 \,\to\, R_2 - R_1\;}\; \left[\begin{array}{ccc|c} 1 & 1 & 1 & 9 \\ 0 & 1 & 1 & 7 \\ 0 & 1 & 1 & 7 \end{array}\right]\]
\[\left[\begin{array}{ccc|c} 1 & 1 & 1 & 9 \\ 0 & 1 & 1 & 7 \\ 0 & 1 & 1 & 7 \end{array}\right] \;\xrightarrow{\;R_3 \,\to\, R_3 - R_2\;}\; \left[\begin{array}{ccc|c} 1 & 1 & 1 & 9 \\ 0 & 1 & 1 & 7 \\ 0 & 0 & 0 & 0 \end{array}\right]\]
With the \(25\)-hour machine, every choice of \(t\) gives a plan \((x, y, z)\):
\[x = 2, \qquad y = 7 - t, \qquad z = t\]
Question
The workshop makes whole, non-negative numbers of each product. How many plans \((2,\; 7 - t,\; t)\) are possible?
8: \(y = 7 - t \geq 0\) and \(z = t \geq 0\) allow \(t = 0, 1, \dots, 7\), from \((2, 7, 0)\) to \((2, 0, 7)\).
The eight plans \((2,\; 7 - t,\; t)\), one for each \(t\) from \(0\) to \(7\):
\[(2, 7, 0), \quad (2, 6, 1), \quad (2, 5, 2), \quad \dots, \quad (2, 0, 7)\]
Question
Every plan makes 2 of \(x\). A unit of \(y\) earns €40, a unit of \(z\) €50. Which of the eight plans earns the most?
Each step up in \(t\) swaps one \(y\) for one \(z\) and adds €10, so push \(t\) as far as it goes: \(t = 7\), the plan \((2, 0, 7)\). As with Session 5's capacity, a boundary wins. That is the idea behind linear programming.
With as many equations as unknowns, the triangular form still tells the fate:
| Picture | After elimination | Fate |
|---|---|---|
| cross or corner | full triangle: each row pins one more unknown (no row of zeros on the left) | unique |
| parallel or tent | a row \(0 = (\text{non-zero})\), e.g. \(0 = -5\) | none |
| same line or book | a row \(0 = 0\), and no row \(0 = (\text{non-zero})\) | infinitely many |
Question
\(x + 2y = 6\) and \(2x + 4y = 8\). How many solutions does the system have?
| (a) Exactly one | (b) None |
| (c) Infinitely many | (d) Exactly two |
(b): doubling the first equation gives \(2x + 4y = 12\), but the second demands \(2x + 4y = 8\). Same slope, different height: parallel lines.
Elimination says the same: \(R_2 - 2R_1\) leaves \(0 = -4\). And (d) is ruled out, as we saw at the start of the section.
Since Session 3: which price should the roastery charge?
You are ready for the mock exam, and for the maths your degree will lean on.
That’s it for today.
Any Questions?
To round off the tutorial, a voluntary mock exam covers Sessions 1–6.
Session 06 - Systems of Linear Equations | Dr. Tobias Vlćek | Home