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galben [10]
3 years ago
5

The total cost of four pens and seven mechanical pencils is $13.25. The cost of each pencil is .75 cents. Write an equation to f

ind out the cost of each pen
Mathematics
1 answer:
Dovator [93]3 years ago
5 0
X = pens and y = pencils

4x + 7y = 13.25
y = 0.75

4x + 7(0.75) = 13.25 
4x + 5.25 = 13.25
4x = 13.25 - 5.25
4x = 8
x = 8/4
x = 2 <===so each pen cost 2 bucks
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Now we substitute this expression for y in the area equation.

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We take the derivative of A with respect to x.

dA/dx = (1/2)[(x) * d/dx(361 - x^2)^(1/2) + (361 - x^2)^(1/2)]

dA/dx = (1/2)[(x) * (1/2)(361 - x^2)^(-1/2)(-2x) + (361 - x^2)^(1/2)]

dA/dx = (1/2)[(361 - x^2)^(-1/2)(-x^2) + (361 - x^2)^(1/2)]

dA/dx = (1/2)[(-x^2)/(361 - x^2)^(1/2) + (361 - x^2)/(361 - x^2)^(1/2)]

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y^2 = 361/2

y = 19/sqrt(2)

We have maximum area at x = 19/sqrt(2) and y = 19/sqrt(2), or when x = y.
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