How many unique real solutions does the following equation have?
( x 2 − 7 x + 1 1 ) x 2 − 1 9 x + 4 8 = 1
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Let f ( x ) = g ( x ) h ( x ) , where g ( x ) = x 2 − 7 x + 1 1 and h ( x ) = x 2 − 1 9 x + 4 8 . We note that f ( x ) = 1 , when
⎩ ⎪ ⎨ ⎪ ⎧ f ( x ) = 1 h ( x ) f ( x ) = ( − 1 ) 2 n f ( x ) = g ( x ) 0 i.e. g ( x ) = 1 for all real h ( x ) i.e. g ( x ) = − 1 and h ( x ) = even integer i.e. h ( x ) = 0 for all real g ( x ) except 0
Case f ( x ) = 1 h ( x ) :
⟹ g ( x ) x 2 − 7 x + 1 1 x 2 − 7 x + 1 0 ( x − 2 ) ( x − 5 ) = 1 = 1 = 0 = 0
⟹ x = { 2 5 ⟹ h ( 2 ) = 1 4 ⟹ h ( 5 ) = − 2 2 ⟹ f ( 2 ) = 1 ⟹ f ( 5 ) = 1
⟹ g ( x ) x 2 − 7 x + 1 1 x 2 − 7 x + 1 2 ( x − 3 ) ( x − 4 ) = − 1 = − 1 = 0 = 0
⟹ x = { 3 4 ⟹ h ( 3 ) = 0 ⟹ h ( 4 ) = − 1 2 ⟹ f ( 3 ) = 1 ⟹ f ( 4 ) = 1
Case f ( x ) = g ( x ) 0 :
⟹ h ( x ) x 2 − 1 9 x + 4 8 ( x − 3 ) ( x − 1 6 ) = 0 = 0 = 0
⟹ x = { 3 1 6 ⟹ g ( 3 ) = − 1 ⟹ g ( 1 6 ) = 1 5 5 ⟹ f ( 3 ) = 1 ⟹ f ( 1 6 ) = 1
Therefore, there are 5 unique solutions of x = 2 , 3 , 4 , 5 , 1 6 .
As noted by @Parth Sankhe , if a b = 1 for real a and b , then one of three cases applies:
Thus, there are 5 solutions.
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Either ( a n y t h i n g ) 0 , or ( 1 ) a n y t h i n g , or ( − 1 ) e v e n i n t e g e r
Solve the quadratics for these values, and you'll get the answers as 2 , 3 , 4 , 5 , 1 6