Summary of "[중2 일차함수] 모르면 중학교 못 다니는 일차함수 쌩기초 총정리 (한방에 이해)"

Main ideas / lessons

A function pairs each x with a single y; points (x, y) represent these pairs on the coordinate plane.
The graph of a linear function is a straight line. Two distinct points determine the line — plot them and connect.
Slope (m) measures a line’s tilt: m = (increase in y) / (increase in x) = (y2 - y1) / (x2 - x1).
- Positive slope → line rises to the right.
- Negative slope → line falls to the right.
Common line equations:
- Point-slope form: y - b = m(x - a) for a line of slope m through (a, b).
- Slope-intercept form: y = m x + c. If you know m and a point (a, b), then c = b - m*a.
- Line through the origin (0,0) has c = 0, so y = m x.
Intercepts:
- y-intercept is y when x = 0; in y = m x + c the y-intercept is c.
- x-intercept is x when y = 0; solve 0 = m x + c ⇒ x = -c / m (if m ≠ 0).
Linear functions are foundational for exams and for later topics (e.g., quadratics). Practice finding slopes, intercepts, and line equations.

Key formulas

Slope between two points (x1, y1) and (x2, y2):
- m = (y2 - y1) / (x2 - x1)
Point-slope form:
- y - b = m(x - a)
Slope-intercept form:
- y = m x + c
- c = b - m * a (given (a, b) and m)
Intercepts:
- y-intercept = c
- x-intercept = -c / m (when m ≠ 0)

Step-by-step methods

Plot a point (x, y)
- Mark x on the horizontal axis and y on the vertical axis; plot the point.
Draw a line given y = m x + c (or a rule)
- Option A — from the equation: choose two convenient x-values (e.g., x = 0 and x = 1), compute y for each, plot, and connect.
- Option B — from intercepts: plot the y-intercept (x = 0) and the x-intercept (y = 0), then connect.
Compute slope between two points (x1, y1) and (x2, y2)
- Use m = (y2 - y1) / (x2 - x1).
- Keep the subtraction order consistent (same order in numerator and denominator).
- Interpret the sign: positive → up to the right; negative → down to the right.
Find the equation of a line given slope m and point (a, b)
- Use point-slope: y - b = m(x - a), then rearrange to y = m x + c if desired.
- Or substitute into y = m x + c and solve c = b - m*a.
Find intercepts from y = m x + c
- y-intercept = c (value when x = 0).
- x-intercept = -c / m (value when y = 0, provided m ≠ 0).
Graph from two given points
- Compute slope, find c if needed, or plot the two points directly and draw the line.
Special case — line through origin
- If the line passes through (0,0), then c = 0 and the equation is y = m x.

Worked examples

Slope of the line through (-2, 3) and (5, -2):
- m = (-2 - 3) / (5 - (-2)) = -5 / 7
Slope of the line through (-1, 3) and (0, 2):
- m = (2 - 3) / (0 - (-1)) = -1 / 1 = -1
Line with slope 3 through (1, 2) (corrected algebra):
- Method 1 (point-slope): y - 2 = 3(x - 1) ⇒ y = 3x - 3 + 2 ⇒ y = 3x - 1
- Method 2 (slope-intercept): y = 3x + c, substitute (1,2) ⇒ 2 = 3*1 + c ⇒ c = -1 ⇒ y = 3x - 1

(Note: the video contains a subtitle/arithmetic error in one example; use the algebraic formulas above.)

Line through points (-1, 4) and (3, 12):
- m = (12 - 4) / (3 - (-1)) = 8 / 4 = 2
- y = 2x + c; substitute (3,12) ⇒ 12 = 2*3 + c ⇒ c = 6 ⇒ y = 2x + 6
Line through (0,0) and (1,2):
- Slope m = (2 - 0) / (1 - 0) = 2 ⇒ y = 2x

Practical tips

Always compute slope with a consistent subtraction order.

Two points are enough to determine a straight line; choose simple points when possible (e.g., include x = 0 to get the y-intercept).

Use intercepts for quick graphing.

Check algebra carefully when solving for the constant term c.

Practice slope, intercepts, and line equations before moving on to quadratics.

Speakers / sources (as named in subtitles)

Oh Sang-jin (clip reference, 2021.04)
Lee Nak-yon (clip reference, 2021.07)
Producer (mentioned)
Chung Seung-je (“The Master Chung Seung-je” — instructor)
Hong Jin Kyung (student/guest)
“Real Study King” (channel/brand label)
Class Leader (label/encouragement phrase)

Want more?

I can: - Create a one-page printable cheat-sheet with the exact algebraic formulas. - Convert the step-by-step methods into 8–10 practice problems with answers.

If you want either, tell me which one (cheat-sheet or practice problems) and any preferences (difficulty level, number of problems).