   Chapter 14, Problem 8T Mathematical Applications for the ...

11th Edition
Ronald J. Harshbarger + 1 other
ISBN: 9781305108042

Solutions

Chapter
Section Mathematical Applications for the ...

11th Edition
Ronald J. Harshbarger + 1 other
ISBN: 9781305108042
Textbook Problem

Suppose a store sells two brands of disposable razors and the profit for these is a function of their two selling prices. The type 1 razor sells for $x, the type 2 sells for$y, and profit is given by P = 915 x − 30 x 2 − 45 x y + 975 y − 30 y 2 − 3500 Find the selling prices that maximize profit. Find the maximum profit.

To determine

To calculate: The selling prices that maximize profit and the maximum profit where a store sells two brands of disposable razors and the profit for these is a function of their two selling prices. The type 1 razor sells for $x, the type 2 sells for$y and the profit is given by P=915x30x245xy+975y30y23500.

Explanation

Given Information:

A store sells two brands of disposable razors and the profit for these is a function of their two selling prices. The type 1 razor sells for $x, the type 2 sells for$y and the profit is given by P=915x30x245xy+975y30y23500.

Formula used:

To calculate relative maxima and minima of the z=f(x,y),

(1) Find the partial derivatives zx and zy.

(2) Find the critical points, that is, the point(s) that satisfy zx=0 and zy=0.

(3) Then find all the second partial derivatives and evaluate the value of D at each critical point, where D=(zxx)(zyy)(zxy)2=2zx22zy2(2zxy)2.

(a) If D>0, then relative minimum occurs if zxx>0 and relative maximum occurs if zxx<0.

(b) If D<0, then neither a relative maximum nor a relative minimum occurs.

For a function f(x,y), the partial derivative of f with respect to x is calculated by taking the derivative of f(x,y) with respect to x and keeping the other variable y constant and the partial derivative of f with respect to y is calculated by taking the derivative of f(x,y) with respect to y and keeping the other variable x constant. The partial derivative of f with respect to x is denoted by fx and with respect to y is denoted by fy.

For a function z(x,y), the second partial derivative,

1. When both derivatives are taken with respect to x is zxx=2zx2=x(zx).

2. When both derivatives are taken with respect to y is zyy=2zy2=y(zy).

3. When first derivative is taken with respect to x and second derivative is taken with respect to y is zxy=2zyx=y(zx).

4. When first derivative is taken with respect to y and second derivative is taken with respect to x is zyx=2zxy=x(zy).

Power of x rule for a real number n is such that, if f(x)=xn then f(x)=nxn1.

Chain rule for function f(x)=u(v(x)) is f(x)=u(v(x))v(x).

Constant function rule for a constant c is such that, if f(x)=c then f(x)=0.

Coefficient rule for a constant c is such that, if f(x)=cu(x), where u(x) is a differentiable function of x, then f(x)=cu(x).

Calculation:

Consider the problem, the type 1 razor sells for $x, the type 2 sells for$y, and the profit is given by P=915x30x245xy+975y30y23500.

The provided function is P(x,y)=915x30x245xy+975y30y23500.

Use the power of x rule for derivatives, the constant function rule, the chain rule, and the coefficient rule,

Thus,

Px=091530(2x)45y=045y=91560xy=18312x9

And,

Py=097530(2y)45x=060y=97545xy=1959x12

Now, calculate the values of x and y

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