Digital SAT Math: using the Desmos calculator well

The rules: what College Board says

• A calculator is allowed only in the Math section. You may not use one in Reading and Writing.
• The Desmos calculator in Bluebook can be switched between graphing and scientific mode at any point in the Math section.
• You may bring your own calculator if it is an approved model: calculators with CAS, QWERTY keyboards and phone calculator apps are not allowed.
• If your calculator fails during the test and you have no backup, you continue with the built-in Desmos.
• On the test, images, folders and notes are turned off in the graphing calculator, and Log mode switches on automatically for exponential, logarithmic and power regressions.
• Every test with math questions also has a reference sheet of common formulas.

Equations and systems: read the answer off the graph

• Type a one-variable equation (say 3x + 5 = 7x - 11) as it is: Desmos shows the answer as the vertical line x = 4.
• For a system, type each equation on its own line and tap the intersection: its coordinates (x, y) appear.
• For "how many solutions" questions, look at the graph: parallel lines mean no solution, the same line means infinitely many, one crossing means one solution.
• For a quadratic, tap the grey points on the curve: the zeros, the vertex and the y-intercept show with their coordinates.
• Type an inequality (y < 2x + 1) and Desmos shades the solution region, so you can see at once whether a point lies in it.

Regression: fit a line to a table

When a data question gives you a table, add a table in Desmos and enter the values in the x₁ and y₁ columns. On the next line write y₁ and mx₁ + b joined by the tilde sign instead of an equals sign: Desmos works out m, b and r².

• Linear model: y₁ tilde mx₁ + b.
• Exponential model: y₁ tilde a · b^x₁ (b to the power x₁). On the test, Log mode switches on here by itself.
• Quadratic model: y₁ tilde ax₁² + bx₁ + c.
• When the question says "line of best fit" or "model", regression saves the arithmetic, but compare with the answer choices: their coefficients may be rounded.

Sliders and function notation

• Type a letter coefficient (y = kx + 3) and Desmos offers to add a slider. Drag it to see for which k the graph takes the shape you need, a quick way to check parameter questions.
• Define f(x) = x² - 4x + 1, then type f(3) or f(a + 1) on the next line to get the value straight away.
• If a trigonometry question uses degrees, check the angle unit under the wrench icon and pick Degrees if needed: a sine worked out in radians gives a different answer.
• If an intersection is off screen, use the zoom buttons or set the window limits under the wrench icon.

When to leave Desmos closed

• Simple arithmetic (adding fractions, percentages, unit changes) is faster in your head or on scratch paper.
• When the answer is an expression ("which expression is equivalent to..."), graphing is slow. Plugging one number in for x and comparing the choices is often quicker.
• On grid-ins Desmos gives a decimal: enter the answer according to the entry rules (see /en/articles/sat-math-topics).
• Do not meet Desmos for the first time on test day. Practise on the official practice tests in Bluebook and on the SAT version of the calculator on the desmos.com testing page.

Practising with Desmos on Trylab

Trylab's SAT Math (/en/sat/math) has the Desmos graphing calculator built into the test, with a scientific calculator and the reference sheet beside it, as in Bluebook. Every question is explained, the score is on the 200-800 scale and mistakes go into a mistake notebook. You can also work out a score at /en/sat/score-calculator.

The free plan gives you 1 mock and 2 section tests a month plus 10 questions a day. You sign in through the Telegram bot.