Expert: Sherry Wallin - 6/11/2011

Question
Hello,

Trying to solve the following: (Solve for y)

101.6 = 6e^[(-o.10469)(0.5)]+ 6e^[(-0.10536)]+ 6e^[(-0.10681)(1.5)] + 106e^[-(y)(2)]


The answer for y is 0.10808

I calculated it as the following and came up with a different solution of -0.39263


101.6 = 5.69401+5.4+5.11176+106e^[-(y)(2)]

101.6-16.20577 = 106e^[-(y)(2)]

Ln85.39423 = Ln106e^[-(y)(2)]

Ln85.39423 = -2y(2)Ln106e

Ln85.39423 = -y(11.32688)

y= -0.39263

Do you happen to know which part of my equation is incorrect?


Thanks for your help!


Chris

Answer
Chris~

Your first two steps are fine. You make your fatal error when you don't divide both sides by 106 in the 3rd row and you take the ln of both sides prematurely. So this is what your work should have looked like:

101.6 = 5.69401+5.4+5.11176+106e^[-(y)(2)]

101.6-16.20577 = 106e^[-(y)(2)]

85.39423/106 = e^[-(y)(2)]

.80561 = e^-2y

ln(.80561) = ln(-2y)

-0.216155525 = -2y

.108077 = y => rounded off is .10808

Math Prof