Substitute into the line equation to find $ y $: - DNSFLEX

Understanding the Context

What Does It Mean to Substitute in a Line Equation?

Substitution means replacing the variable in one equation with another value or expression—typically when solving for $ y $. In the context of a linear equation, this often involves plugging in a known $ x $-value or replacing one variable to simplify and isolate $ y $. Substitution turns a single equation with two variables into a straightforward calculation for $ y $, which is especially useful when analyzing slope-intercept form or comparing lines.

Why Substitution Matters in Finding $ y $

Key Insights

Linear equations are defined by the relationship $ y = mx + b $, where:

• $ m $ is the slope (rate of change),
  
• $ b $ is the y-intercept (value of $ y $ when $ x = 0 $).

However, real-world problems or systems of equations may present equations in different forms:

• $ x + 2y = 10 $ (standard form),
  
• $ y = 3x - 5 $ (slope-intercept form),
  
• or a general equation needing rearrangement.

Substitution lets you transform these into clear expressions for $ y $, enabling quick graphical interpretation and precise numerical solutions.

Step-by-Step: Solving for $ y $ Using Substitution

Final Thoughts

Example Problem:
Use substitution to find $ y $ when $ x = 2 $ in the equation $ 3x + 4y = 20 $.

Step 1: Start with the equation
$$ 3x + 4y = 20 $$

Step 2: Substitute the given $ x $-value
Replace $ x $ with 2:
$$ 3(2) + 4y = 20 $$
$$ 6 + 4y = 20 $$

Step 3: Solve for $ y $
Subtract 6 from both sides:
$$ 4y = 14 $$
Divide by 4:
$$ y = rac{14}{4} = rac{7}{2} $$

Result: When $ x = 2 $, $ y = rac{7}{2} $. This gives a precise point $ (2, rac{7}{2}) $ on the line.