Basic Concepts Involved in Factoring Trinomials

If two numbers multiply to give a positive number, then they must both be positive or they must both be negative. That is, two numbers that multiply to a positive number must have the same sign.

If two numbers multiply to give a negative number, then one must be positive, and the other must be negative.

When you add two numbers that have the same sign, then in your head you do an addition problem.

For example, to mentally compute the sum \((-2)+(-5)\), in your head you would compute \(2+5\), and then assign a negative sign to your answer.

Think of the number line interpretation of this fact.

• Start at zero.
• You must walk in only one direction (both numbers have the same sign).
• Each time you walk, you are getting farther from zero.
• Your final distance from zero is the sum of the individual distances you walk.

When you add two numbers that have different signs, then in your head you do a subtraction problem.

For example, to mentally compute \(-5+2\), you would think \(5-2\), and then make the answer negative.

Think of the number line interpretation of this fact.

• Start at zero.
• You must walk in both directions (the numbers have different signs).
• So, you will be doing some back-tracking (some overlapping).
• Your final distance from zero is the difference of the individual distances you walk.

If two numbers have the same sign and their sum is positive, then both numbers must be positive.

For example, if two numbers have the same sign and add to \(10\), then they must both be positive.

Think of the number line interpretation of this fact.

• Start at zero.
• You need to walk in only one direction (both numbers have the same sign).
• You need to end up to the right of zero (the sum is positive).
• So, you must walk to the right both times (both numbers must be positive).

If two numbers have the same sign and their sum is negative, then both numbers must be negative.

For example, if two numbers have the same sign and add to \(-10\), then they must both be negative.

Think of the number line interpretation of this fact.

• Start at zero.
• You need to walk in only one direction (both numbers have the same sign).
• You need to end up to the left of zero (the sum is negative).
• So, you must walk to the left both times (both numbers must be negative).

If two numbers have different signs and their sum is positive, then the bigger number must be positive. (Remember that bigger means farther away from zero).

For example, if two numbers have different signs and add to \(10\), then the bigger number must be positive.

Think of the number line interpretation of this fact.

• Start at zero.
• You must walk in both directions (the numbers have different signs).
• You need to end up to the right of zero (the sum is positive).
• So, you must walk farther to the right (the bigger number must be positive).

If two numbers have different signs and their sum is negative, then the bigger number must be negative. (Remember that bigger means farther away from zero).

For example, if two numbers have different signs and add to \(-10\), then the bigger number must be negative.

• Start at zero.
• You must walk in both directions (the numbers have different signs).
• You need to end up to the left of zero (the sum is negative).
• So, you must walk farther to the left (the bigger number must be negative).