Exponential Equation

Calculus Level 3

4 x + 5 x = 6 x \large 4^x + 5^x = 6^x

Find the value of x x satisfying the equation above.

Give your answer to 3 decimal places.

1.996 2.488 3.005 2.781

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2 solutions

展豪 張
May 7, 2016

I used Newton's method and a programme to solve this question.
Let f ( x ) = 6 x 5 x 4 x f(x)=6^x-5^x-4^x .
f ( x ) = ( ln 6 ) 6 x ( ln 5 ) 5 x ( ln 4 ) 4 x f'(x)=(\ln 6)6^x-(\ln 5)5^x-(\ln 4)4^x
We are finding x x for f ( x ) = 0 f(x)=0
Here we can use iteration a n + 1 = a n f ( a n ) f ( a n ) a_{n+1}=a_n-\dfrac{f(a_n)}{f'(a_n)} which is the Newton's method.
Pick some initial x x for the iteration to converges.
We can obtain the answer is 2.487939 2.488 2.487939\cdots\approx 2.488

Sabhrant Sachan
May 7, 2016

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 , 16 + 25 = 41 > 36 At x = 3 , 64 + 125 = 189 < 216 The inequality has flipped , There must be a Solution between 2 and 3 At x = 2.5 we have , 2 5 + 5 5 2 6 5 2 2 5 + 2.23 6 5 2.4 5 5 87.9 < 88.1 From the Options , The Answer will be : 2.488 \text {It can be clearly Seen that for }x\le0 , 6^x<4^x+5^x \\ \text {and For } x\ge4 , 6^x>4^x+5^x \\ \text{we have to check for } x=1,2,3 \\ \text{At }x=1, 4+5=9>6 \\ \text{At }x=2, 16+25=41>36 \\ \text{At }x=3, 64+125=189<216 \\ \text{The inequality has flipped , There must be a Solution between 2 and 3 } \\ \text{At }x=2.5 \text{ we have ,} \\ \implies 2^5+5^{\frac52} \boxed{}6^{\frac52} \\ \implies 2^5+2.236^5\boxed{}2.45^5 \\ \implies 87.9<88.1 \\ \text{From the Options , The Answer will be : } \boxed{2.488}

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