x − 1 1 − x − 2 4 + x − 3 4 − x − 4 1 < 3 0 1
If the range of x satisfying the above inequality is in the form as given below.
( − ∞ , a ) ∪ ( b , c ) ∪ ( d , e ) ∪ ( f , g ) ∪ ( h , ∞ )
Find a + b + c + d + e + f + g + h .
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Typo after the line you put back t: t + 2 = ( x − 2 ) ( x − 3 )
Was too lazy to factorize :p Anyway, good solution. Could also be explained using the Wavy curve method though
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Drawing the number line is exactly the same.... Curve is drawn in mind to plot sign scheme on number line.... :-)
Call the given function f ( x ) .
Notice that f ( x ) = f ( 5 − x ) , which gives h = 5 − a , g = 5 − b , f = 5 − c and e = 5 − d .
So a + b + c + d + e + f + g + h = ( a + h ) + ( b + g ) + ( c + f ) + ( d + e ) = 5 + 5 + 5 + 5 = 2 0
Oh, that's a nice trick. You have made use of the symmetries in the problem statement to find the answer without finding the actual range of the function. I have upvoted your solution (+1)
Congratulations on writing your first solution on Brilliant, and thanks for sharing your approach :)
The exact same method for me too.
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Relevant wiki: Polynomial Inequalities Problem Solving - Basic
( x − 3 4 − x − 2 4 ) + ( x − 1 1 − x − 4 1 ) < 3 0 1 ⟹ ( x 2 − 5 x + 4 ) + 2 ( x − 2 ) ( x − 3 ) 4 − x 2 − 5 x + 4 ( x − 1 ) ( x − 4 ) 3 < 3 0 1
Put x 2 − 5 x + 4 = t
⟹ t + 2 4 − t 3 < 3 0 1 ⟹ t ( t + 2 ) t − 6 < 3 0 1 ⟹ t ( t + 2 ) t 2 − 2 8 t + 1 8 0 > 0 ⟹ t ( t + 2 ) ( t − 1 0 ) ( t − 1 8 ) > 0 Put t back and factorise further to obtain:
⟹ ( x − 1 ) ( x − 4 ) ( x − 2 ) ( x − 3 ) ( x + 1 ) ( x + 2 ) ( x − 6 ) ( x − 7 ) > 0
Plot a number line to see solution set as:-
x ∈ ( − ∞ , − 2 ) ∪ ( − 1 , 1 ) ∪ ( 2 , 3 ) ∪ ( 4 , 6 ) ∪ ( 7 , ∞ )
Hence,
− 2 − 1 + 1 + 2 + 3 + 4 + 6 + 7 = 2 0