I don't want Integers

Calculus Level 4

x 2 4 x + 5 = 2 x 2 x^2 - 4x + 5 = 2^{x-2}

What is the approximate real non-integral solution of the equation above? Round your answer to two decimal places.

Details and Assumptions

  • Do not use a graphical calculator to solve this problem.


The answer is 6.25.

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2 solutions

Chew-Seong Cheong
Jan 13, 2015

We note that x 2 4 x + 5 = 2 x 2 ( x 2 ) 2 + 1 = 2 x 2 \space x^2-4x+5 = 2^{x-2} \quad \Rightarrow (x-2)^2+1 = 2^{x-2}

Let y = x 2 f ( y ) = y 2 + 1 2 y = 0 \space y=x-2\quad \Rightarrow f(y) = y^2 +1 -2^y = 0

f ( y ) d f ( y ) d y = 2 y ln 2 ˙ 2 y \quad \Rightarrow f'(y)\frac {df(y)}{dy} = 2y - \ln{2}\dot{}2^y

We note that: f ( 4 ) = 1 f(4) = 1\space and f ( 5 ) = 6 \space f(5) = -6\space implying there is a root within 4 < y < 5 4 < y < 5 . Using Newton's method, we have:

y n + 1 = y n f n ( y ) f n ( y ) y_{n+1} = y_n - \dfrac {f_n(y)}{f_n'(y)}

y n f ( y n ) f ( y n ) f ( y n ) f ( y n 4 1 3.090354889 0.323587431 4.323587431 0.329608244 5.231722584 0.063001858 4.260585573 0.014848109 4.764684139 0.003116284 4.257469289 3.49722 × 1 0 05 4.742249655 7.3746 × 1 0 06 4.257461914 1.95488 × 1 0 10 4.742196638 4.12231 × 1 0 11 \begin {matrix} y_n & f(y_n) & f'(y_n) & \frac {f(y_n)}{f'(y_n} \\ 4&1 & -3.090354889&-0.323587431 \\ 4.323587431 & -0.329608244& -5.231722584& 0.063001858\\ 4.260585573 &-0.014848109& -4.764684139& 0.003116284\\ 4.257469289 &-3.49722\times 10^{-05} & -4.742249655& 7.3746\times 10^{-06}\\ 4.257461914 &-1.95488\times 10^{-10}& -4.742196638&4.12231\times 10^{-11} \end {matrix}

Therefore, y 4.257461914 x = y + 2 6.26 y \approx 4.257461914 \quad \Rightarrow x = y+2 \approx \boxed{6.26} .

William Isoroku
Jan 20, 2015

Just graph it as 2 functions.

Yeah, I agree with you .

A Former Brilliant Member - 6 years, 4 months ago

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