Finding Values for a and b: Tangent and Perpendicularity in y=ax^3 and y=2x+3

In summary, to find the values of a and b for the given information, we use the fact that the tangent line to the curve y=ax^3 at the point (-1,b) is perpendicular to y = 2x+3. From this, we can determine that the slope of the tangent line is equal to the derivative of y=ax^3, which is 3ax^2. Since the slope of y = 2x+3 is 2, we can set these equal to each other and solve for a, which is found to be -1/6. Substituting this value back into y=ax^3, we can solve for b, which is found to be 1/6.
  • #1
Sirsh
267
10
The tangent to the curve y=ax^3 at the point (-1,b) is perpendicular to the line y = 2x+3. Find the values of a and b.

Could someone show me how to answer that?
 
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  • #2
First, if (-1, b) is on the curve [itex]y= ax^3[/itex], what is b?

The two lines given by y= mx+ u and y= nx+ v are perependicular if and only if mn= -1. With y= 2x+ 3, m= 2 so what is n?

The slope, n, of a tangent line is equal to the derivative of the function at that point. What is the derivative there. What is the derivative of [itex]y= ax^3[/itex] at x= -1?
 
  • #3
y = 2x+3
y' = 2
m = 2
so, a perpendicular gradient to 2 is -1/2.
y = ax^3
y' = 3ax^2
-1/2 = 3ax^2
-1/2 = 3a(-1)^2
a = -1/6

y = -1/6(-1)^3
b = 1/6.
 
  • #4
Exactly right! Congratulations.
 

What is simple differentiation?

Simple differentiation is a mathematical process used to find the rate of change of a function at a specific point. It involves finding the slope of the tangent line to the function at that point.

What is the purpose of simple differentiation?

The purpose of simple differentiation is to calculate the instantaneous rate of change of a function at a specific point. This can be useful in various applications, such as finding the velocity of an object at a particular time or the growth rate of a population.

What is the difference between simple differentiation and complex differentiation?

Simple differentiation involves finding the rate of change at a single point, while complex differentiation involves finding the rate of change over a range of values. Simple differentiation uses basic differentiation rules, while complex differentiation may require the use of more advanced techniques.

What are some common applications of simple differentiation?

Simple differentiation is commonly used in physics, economics, and other fields to analyze and model real-world phenomena. It can be used to calculate velocity, acceleration, growth rates, and other important quantities.

How do I perform simple differentiation?

To perform simple differentiation, you can use the power rule, product rule, quotient rule, or chain rule, depending on the function you are differentiating. You can also use tables or graphs to help visualize the process. It is important to have a strong understanding of basic algebra and calculus principles before attempting simple differentiation.

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