Interesting Maths Problem. Find the mistake.......?

[ShreyaA]

I got a problem :

Solve for x : 2 ?( x + 7 ) + 10 = 2 x

I proceeded in the following manner ---

2 ?( x + 7 ) + 10 = 2 x

=> 2 ?( x + 7 ) = 2 x - 10

Squaring both the sides we get --

=> 4 x² - 40 x - 4 x + 100 - 28 = 0

=> 4 x² - 44 x + 72 = 0

=> x² - 11 x + 18 = 0

Giving x = 2 and 9.
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Now the original eqn is satisfied by 9 but refuses to go with 2 . Why is it so ?
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[danny_lv2003]

to make sure ,you just need to take another step, to test these 2 keys
if X=2
then:2 ?( x + 7 )=6 ,while 2 x - 10= minus 6
So , they happen to be opposite number .
6 is not the right answer
But for 9 , it's okay

 

[Tidus]

2^2 - 2(11) +18 does = 0