4 x + 5 x = 6 x
Find the value of x satisfying the equation above.
Give your answer to 3 decimal places.
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It can be clearly Seen that for x ≤ 0 , 6 x < 4 x + 5 x and For x ≥ 4 , 6 x > 4 x + 5 x we have to check for x = 1 , 2 , 3 At x = 1 , 4 + 5 = 9 > 6 At x = 2 , 1 6 + 2 5 = 4 1 > 3 6 At x = 3 , 6 4 + 1 2 5 = 1 8 9 < 2 1 6 The inequality has flipped , There must be a Solution between 2 and 3 At x = 2 . 5 we have , ⟹ 2 5 + 5 2 5 6 2 5 ⟹ 2 5 + 2 . 2 3 6 5 2 . 4 5 5 ⟹ 8 7 . 9 < 8 8 . 1 From the Options , The Answer will be : 2 . 4 8 8
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I used Newton's method and a programme to solve this question.
Let f ( x ) = 6 x − 5 x − 4 x .
f ′ ( x ) = ( ln 6 ) 6 x − ( ln 5 ) 5 x − ( ln 4 ) 4 x
We are finding x for f ( x ) = 0
Here we can use iteration a n + 1 = a n − f ′ ( a n ) f ( a n ) which is the Newton's method.
Pick some initial x for the iteration to converges.
We can obtain the answer is 2 . 4 8 7 9 3 9 ⋯ ≈ 2 . 4 8 8