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Algebra Level 2

The equation

x 10 = 7 | x - 10 | = 7

is equivalent to the equation

x 2 + a x + b = 0. x^2 + ax + b = 0.

What is the value of a + b a+b ?


The answer is 31.

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9 solutions

Daniel Chiu
Oct 6, 2013

The first equation means that x 10 = ± 7 x-10=\pm 7 . We can square this equation and keep all solutions: x 2 20 x + 100 = 49 x^2-20x+100=49 x 2 20 x + 51 = 0 x^2-20x+51=0 The answer is 20 + 51 = 31 -20+51=\boxed{31} .

NOTE: There will be the same number of solutions since an absolute value equation has 2 solutions if the RHS is positive

can i know.why must we square the x-10=7?

Munirah Mahadzan - 7 years, 8 months ago

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We square it to get a quadratic, which is what the question asks for.

Daniel Chiu - 7 years, 8 months ago

|x-10| = 7

tells that x = 3 , x = 17

making it as a quadratic equation

(x - 17) (x - 3) gives

x^2 - 20x + 51

a = -20 b = 51

a+b = 31

Daniel Gong
Oct 6, 2013

Absolute value of (x-10)=7 is also equal to (x-10)^2 = 49. Now the problem can be expanded so x^2-20x+100=49. The equation simplifies to x^2-20x+51. So your a and b are -20 and 51, which gets you answer of 31

|x-10|=7 |17-10|=7 --> 1 solution |3-10|=7 ---> 2 Solution

{Since the next equation is quadratic , we need two solutions}

Substituting , we get 289+17a+b=0 and 9-3a+b=0

Solving them , we arrive at a=-21 and b =50

therefore -21+50 = 31 .

Vinayak Rastogi
Mar 1, 2014

quite easy,,,,, first u had to find out the possible values of x from equation 1 which were - 3 and 17. then , as its given that the equations are equivalent, put the same values of x one by one in equation 2, then u will obtain a pair of linear equations in 2 variables. after solving them the values of a and b come out to be - (-20) and 51 respectively . therefore the answer comes out to be 31

its +3 and 17

Vighnesh Raut - 7 years, 2 months ago
Justin Wong
Oct 8, 2013

Equivalent equations have equivalent roots, so start by finding roots of the first equation. Splitting the absolute value function into separate equations, x 10 = 7 x-10=7 and x 10 = 7 x-10=-7 , so x = 17 , 3 x=17,3 . These roots can be expressed as ( x 17 ) = 0 (x-17)=0 and ( x 3 ) = 0 (x-3)=0 . Multiplying together yields ( x 17 ) ( x 3 ) = 0 (x-17)(x-3)=0 , the factored form of a quadratic. The problem asks for coefficients in standard form though, so expanding the latter form yields x 2 20 x + 51 = 0 x^2-20x+51=0 . The coefficients in question are 20 -20 and 51 51 , so the answer is 20 + 51 = 31 -20+51=31 .

Oon Han
Dec 9, 2018

x 10 = 7 |x - 10| = 7 Squaring the equation gets rid of the absolute value, giving: x 2 20 x + 100 = 49 x 2 20 x + 51 = 0 \begin{aligned} x^2 - 20x + 100 &= 49 \\ x^2 - 20x + 51 &= 0 \end{aligned} The sum of the coefficients of the x x and constant terms is 20 + 51 = 31 -20 + 51 = \boxed{31} .

Therefore, the answer is 31 .

Harish Kp
Feb 26, 2014

from the first equation....x=3,17 substitute x in second equation..... 289 + 17a+b=0 ........(1) 9+3a+b=0.......(2) subtract ....(1) minus (2)....we will get a= -20 substitute a in any one of equation....we will get b = 51 .....so a+b=31

Please write your solutions neatly and use LaTeX \LaTeX . @Harish Kp

Anuj Shikarkhane - 6 years, 7 months ago
Dee Masbro
Oct 11, 2013

Very easy, x 10 = 7 |x-10|=7 has two solution , which is 3 3 OR 17 17 .

Cause x 10 = 7 |x-10|=7 equivalent x 2 + a x + b = 0 x^2+ax+b=0 so,

we can substitute x = 3 x=3 to x 2 + a x + b = 0 x^2+ax+b=0 and we got

3 a + b = 9 3a+b=-9

now subtitute x = 17 x=17 , we got

17 a + b = 289 17a+b=-289

by system equation two variable , we got

a = 20 a=-20 , b = 51 b=51 and the answer is 20 + 51 -20+51 --> 31 31

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