Solve: Adding Negative Decimals (-0.21) + (-0.79)

Q: Why do I add the numbers when the operation is addition?

Great observation! When you see $ (-0.21) + (-0.79) $, you're adding two negative numbers. Think of it as "I owe 21 cents plus I owe 79 cents" - together I owe 1 dollar, so the answer is negative.

Q: How is this different from subtraction?

Addition of negatives like $ (-0.21) + (-0.79) $ means you're combining debts. Subtraction like $ (-0.21) - (0.79) $ would mean subtracting a positive, which makes the result even more negative!

Q: Can I just ignore the parentheses?

Never ignore parentheses! They show that the entire number is negative. $ (-0.21) + (-0.79) $ is very different from $ -0.21 + -0.79 $, even though they mean the same thing.

Q: What if the decimals don't add up to a whole number?

No problem! For example, $ (-0.15) + (-0.23) = -0.38 $. Just add the decimal parts normally: 0.15 + 0.23 = 0.38, then make the result negative.

Q: How do I line up the decimal points when adding?

Stack them vertically with decimal points aligned:0.21+ 0.79= 1.00Then apply the negative sign to get -1.00.

Adding Negative Decimals with Same Signs

$(-0.21)+(-0.79)=$

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations

00:00 Solution

00:03 Let's take the point (-0.21) on the axis

00:06 Positive times negative is always negative

00:09 Therefore subtract, move to the left (negative)* on the axis

00:16 And this is the solution to the question

Step-by-step written solution

Follow each step carefully to understand the complete solution

Understand the problem

$(-0.21)+(-0.79)=$

Step-by-step solution

Using the following rule:

$+(-x)=-x$

Let's rewrite the exercise as seen below:

$-0.21-0.79=$

Since we are subtracting from a negative number, the result will be negative.

Therefore:

$-0.21\\-0.79\\-1 .00$

Final Answer

$-1$

Key Points to Remember

Essential concepts to master this topic

Rule: When adding two negative numbers, add their absolute values
Technique: Rewrite (-0.21) + (-0.79) as -(0.21 + 0.79) = -1.00
Check: Verify 0.21 + 0.79 = 1.00, so answer is -1.00 ✓

Common Mistakes

Avoid these frequent errors

Forgetting the negative sign in the final answer
Don't just add 0.21 + 0.79 = 1 and forget the negative sign! This gives a positive result when both numbers are negative. Always remember that adding two negative numbers gives a negative result.

Practice Quiz

Test your knowledge with interactive questions

a is negative number.

b is negative number.

What is the sum of a+b?

FAQ

Everything you need to know about this question

Why do I add the numbers when the operation is addition?

Great observation! When you see $(-0.21) + (-0.79)$ , you're adding two negative numbers. Think of it as "I owe 21 cents plus I owe 79 cents" - together I owe 1 dollar, so the answer is negative.

How is this different from subtraction?

Addition of negatives like $(-0.21) + (-0.79)$ means you're combining debts. Subtraction like $(-0.21) - (0.79)$ would mean subtracting a positive, which makes the result even more negative!

Can I just ignore the parentheses?

Never ignore parentheses! They show that the entire number is negative. $(-0.21) + (-0.79)$ is very different from $-0.21 + -0.79$ , even though they mean the same thing.

What if the decimals don't add up to a whole number?

No problem! For example, $(-0.15) + (-0.23) = -0.38$ . Just add the decimal parts normally: 0.15 + 0.23 = 0.38, then make the result negative.

How do I line up the decimal points when adding?

Stack them vertically with decimal points aligned:

0.21
+ 0.79
= 1.00

Then apply the negative sign to get -1.00.

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