Find the Ascending Area of f(x)=-3x²+12: Quadratic Function Analysis

Quadratic Functions with Derivative Analysis

Find the ascending area of the function

f(x)=3x2+12 f(x)=-3x^2+12

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Find the domain of increase of the function
00:04 Let's examine the trinomial coefficients
00:08 Let's look at the coefficient of X squared, negative so the function is concave down
00:16 We'll use the formula to find the vertex of the parabola
00:21 We'll substitute appropriate values according to the given data and solve to find the vertex
00:25 This is the X value at the vertex of the parabola
00:28 Let's determine when the parabola is decreasing and when it's increasing based on its type
00:31 Let's draw the X-axis and find the domain of increase
00:38 And this is the solution to the question

Step-by-step written solution

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1

Understand the problem

Find the ascending area of the function

f(x)=3x2+12 f(x)=-3x^2+12

2

Step-by-step solution

Let's solve the problem.

Step 1: Calculate the derivative of the function:

The function is f(x)=3x2+12 f(x) = -3x^2 + 12 .

The derivative f(x) f'(x) is calculated using the power rule:

f(x)=ddx(3x2+12)=6x f'(x) = \frac{d}{dx}(-3x^2 + 12) = -6x .

Step 2: Find where the derivative is positive:

To find the interval where the function is increasing, solve the inequality f(x)>0 f'(x) > 0 .

This yields:

6x>0-6x > 0

Divide both sides by 6-6 (remember to reverse the inequality sign when dividing by a negative):

x<0 x < 0

Therefore, the function is increasing for x<0 x < 0 .

Thus, the ascending area of the function is x<0 x < 0 .

3

Final Answer

x<0 x < 0

Key Points to Remember

Essential concepts to master this topic
  • Rule: Function increases when derivative is positive f'(x) > 0
  • Technique: Calculate f'(x) = -6x, then solve -6x > 0
  • Check: Test x = -1: f'(-1) = 6 > 0, so increasing ✓

Common Mistakes

Avoid these frequent errors
  • Forgetting to flip inequality sign when dividing by negative
    Don't divide -6x > 0 by -6 and keep the same inequality sign = x > 0! This gives the wrong interval where the function is actually decreasing. Always flip the inequality sign when dividing or multiplying by a negative number.

Practice Quiz

Test your knowledge with interactive questions

Find the ascending area of the function

\( f(x)=2x^2 \)

FAQ

Everything you need to know about this question

What does 'ascending area' mean in math?

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The ascending area (or increasing interval) is where the function's values get larger as x increases. It's like going uphill on the graph!

Why do I need to find the derivative first?

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The derivative f(x) f'(x) tells you the slope at any point. When f(x)>0 f'(x) > 0 , the slope is positive, meaning the function is going upward.

How do I remember when to flip the inequality sign?

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Memory trick: When you divide or multiply both sides by a negative number, always flip the inequality! Think of it as the negative 'flipping' everything around.

Can I just look at the graph instead of using calculus?

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Yes! For this parabola opening downward, you can see it increases on the left side of the vertex. But using derivatives gives you the exact mathematical proof.

What if the derivative equals zero?

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When f(x)=0 f'(x) = 0 , the function has a critical point - it's neither increasing nor decreasing at that instant. Here, that happens at x = 0.

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