گام به گام تمرین صفحه 21 درس معادلۀ درجۀ دوم ریاضی و آمار (1)

\(2{x^2} - 8 = 0 \Rightarrow 2{x^2} = 8 \Rightarrow {x^2} = 4 \Rightarrow x = \pm 2\)

\(\begin{array}{l}(x + 2)(x - 3) = x - 3 \Rightarrow (x + 2)(x - 3) - (x - 3) = 0\\\\ \Rightarrow (x - 3)(x + 2 - 1) = 0 \Rightarrow (x - 3)(x + 1) = 0\\\\ \Rightarrow \left\{ \begin{array}{l}x = - 1\\\\x = 3\end{array} \right.\end{array}\)

\({x^4} - 2{x^2} = 0 \Rightarrow {x^2}({x^2} - 2) = 0 \Rightarrow \left\{ \begin{array}{l}{x^2} = 0 \Rightarrow x = 0\\\\{x^2} = 2 \Rightarrow x = \pm \sqrt 2 \end{array} \right.\)

\(\begin{array}{l}{x^2} = x - \frac{1}{4} \Rightarrow {x^2} - x + \frac{1}{4} = 0 \Rightarrow {(x - \frac{1}{2})^2} = 0\\\\ \Rightarrow x - \frac{1}{2} = 0 \Rightarrow x = \frac{1}{2}\end{array}\)

\(2{x^2} - 8x = 0 \Rightarrow 2x(x - 4) = 0 \Rightarrow \left\{ \begin{array}{l}x = 0\\\\x = 4\end{array} \right.\)

\({x^2} - 5x + 6 = 0 \Rightarrow (x - 2)(x - 3) = 0 \Rightarrow \left\{ \begin{array}{l}x = 2\\\\x = 3\end{array} \right.\)

\(\begin{array}{l}\frac{{{x^2}}}{3} = x \Rightarrow {x^2} = 3x \Rightarrow {x^2} - 3x = 0 \Rightarrow x(x - 3) = 0\\\\ \Rightarrow \left\{ \begin{array}{l}x = 0\\\\x = 3\end{array} \right.\end{array}\)

\(\begin{array}{l}{x^2} = 5 - {x^2} \Rightarrow 2{x^2} = 5 \Rightarrow {x^2} = \frac{5}{2} \Rightarrow x = \pm \sqrt {\frac{5}{2}} \\\\ \Rightarrow x = \pm \frac{{\sqrt {10} }}{2}\end{array}\)

\({x^2} + 4x + 4 = 0 \Rightarrow {(x + 2)^2} = 0 \Rightarrow x + 2 = 0 \Rightarrow x = - 2\)

\(9{x^2} + 3x - 2 = 0 \Rightarrow (3x + 2)(3x - 1) = 0 \Rightarrow \left\{ \begin{array}{l}x = \frac{1}{3}\\\\x = - \frac{2}{3}\end{array} \right.\)

\({(x - 3)^2} = 4 \Rightarrow x - 3 = \pm 2 \Rightarrow \left\{ \begin{array}{l}x - 3 = 2 \Rightarrow x = 5\\\\x - 3 = - 2 \Rightarrow x = 1\end{array} \right.\)

\( = {6^2} = 36\) مساحت مربع

مساحت بریده شده برابر است با:

\( = {S_1} = {x^2}\) مساحت مربع کوچک

\( = {S_2} = x \times 2x = 2{x^2}\) مساحت مستطیل

\( = {S_3} = \frac{{\sqrt 2 x \times \sqrt 2 x}}{2} = {x^2}\) مساحت مثلث

\({S_1} + {S_2} + {S_3} = {x^2} + 2{x^2} + {x^2} = 4{x^2}\)

\( = 36 - 24 = 12 = 4{x^2}\) مساحت قسمت بریده شده

\( \Rightarrow {x^2} = 3 \Rightarrow x = \pm \sqrt 3 \)

\(x = - \sqrt 3 \) غیر قابل قبول است چون اندازه ضلع منفی نمی شود. بنابراین طول ضلع کوچک بریده شده برابر \(x = \sqrt 3 \) می باشد.

\(\begin{array}{l}\left. \begin{array}{l}x = 2 \Rightarrow x - 2 = 0\\\\x = - 3 \Rightarrow x + 3 = 0\end{array} \right\} \Rightarrow (x - 2)(x + 3) = 0\\\\ \Rightarrow {x^2} + x - 6 = 0\end{array}\)

\(x = 1 \Rightarrow x - 1 = 0 \Rightarrow {(x - 1)^2} = 0 \Rightarrow {x^2} - 2x + 1 = 0\)

\(\begin{array}{l}{(x - 1)^2} = K \Rightarrow x - 1 = \pm \sqrt K \Rightarrow x = 1 \pm \sqrt K \\\\K = 2 \Rightarrow x = 1 \pm \sqrt 2 \Rightarrow \left\{ \begin{array}{l}x = 1 + \sqrt 2 \\\\x = 1 - \sqrt 2 \end{array} \right.\\\\K = 4 \Rightarrow x = 1 \pm \sqrt 4 = 1 \pm 2 \Rightarrow \left\{ \begin{array}{l}x = 3\\\\x = - 1\end{array} \right.\\\\K = 0 \Rightarrow x = 1 \pm \sqrt 0 \Rightarrow x = 1\\\\K = - 9 \Rightarrow x = 1 \pm \sqrt { - 9} \end{array}\)

به ازای K=-9 جواب ندارد.

به ازای K=0

به ازای K>0 (مقادیر مثبت K)

به ازای K<0 (مقادیر منفی K)

مساحت مستطیل سفید + مساحت مثلث سفید = مساحت فلش

\( = \frac{1}{2} \times 3x \times 2x + x \times 2x = 5{x^2}\)

هزینه برچسب سفید:

\(5{x^2} \times 30 = 150{x^2}\)

فلش – مستطیل تابلو = مساحت قسمت آبی

\( = 60 \times 40 - 5{x^2} = 2400 - 5{x^2}\)

هزینه برچسب آبی:

\((2400 - 5{x^2}) \times 10 = 24000 - 50{x^2}\)

\( \Rightarrow 150{x^2} + 24000 - 50{x^2} = 27000\) 27000 = مجموع هزینه ها

\(100{x^2} = 3000 \Rightarrow {x^2} = 30 \Rightarrow x = \pm \sqrt {30} \)

\(x = - \sqrt {30} \) غیر قابل قبول است.

\({a^2} = {(x + \frac{x}{2})^2} + {(2x)^2} = \frac{{25}}{4}{x^2} \Rightarrow a = \pm \frac{5}{2}x \Rightarrow a = \frac{5}{2}x\)

\( = 2a + 3x + 2(2x) = 2(\frac{5}{2}x) + 3x + 4x = 12x\) محیط فلش

\( = 150{x^2} + 24000 - 50{x^2} + 1200x = 34800\) هزینه برچسب زرد

\( = 150{x^2} + 24000 - 50{x^2} + 1200x = 34800\) هزینه رنگ آمیزی تابلو

\(\begin{array}{l} \Rightarrow 10\,0{x^2} + 1200x - 10800 = 0 \Rightarrow {x^2} + 12x - 108 = 0\\\\ \Rightarrow (x + 18)(x - 6) = 0 \Rightarrow x = 6\end{array}\)

جواب x=-18 غیر قابل قبول می باشد.