It is given that 2x×32(2x)8x+1=9(52x)−52x+15x−5x−1\frac{2^x \times 32(2^x)}{8^{x+1}} = \frac{9(5^{2x}) - 5^{2x+1}}{5^x - 5^{x-1}}. Evaluate 10x10^x without using a calculator. Solve x+7−x−1=0\sqrt{x+7} - x - 1 = 0.

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Worked solution

9(a)

1. Simplify the left-hand side (LHS) of the equation by expressing all terms in base 2.

LHS=2x×25×2x(23)x+1\text{LHS} = \frac{2^x \times 2^5 \times 2^x}{(2^3)^{x+1}}
=22x+523x+3= \frac{2^{2x+5}}{2^{3x+3}}
=2(2x+5)−(3x+3)= 2^{(2x+5) - (3x+3)}
=2−x+2= 2^{-x+2}

2. Simplify the right-hand side (RHS) of the equation by factoring out common terms.

RHS=9(52x)−5(52x)5x−15(5x)\text{RHS} = \frac{9(5^{2x}) - 5(5^{2x})}{5^x - \frac{1}{5}(5^x)}
=4(52x)45(5x)= \frac{4(5^{2x})}{\frac{4}{5}(5^x)}
=5(52x−x)= 5(5^{2x - x})
=5(5x)=5x+1= 5(5^x) = 5^{x+1}

3. Equate the simplified LHS and RHS, and rearrange to find $10^x$.

2−x+2=5x+12^{-x+2} = 5^{x+1}
22×2−x=51×5x2^2 \times 2^{-x} = 5^1 \times 5^x
4×12x=5×5x4 \times \frac{1}{2^x} = 5 \times 5^x
45=2x×5x\frac{4}{5} = 2^x \times 5^x
10x=4510^x = \frac{4}{5}

Answer

45\frac{4}{5}

9(b)

1. Isolate the surd and square both sides

x+7=x+1\sqrt{x+7} = x + 1
x+7=(x+1)2x + 7 = (x + 1)^2

2. Expand and form a quadratic equation

x+7=x2+2x+1x + 7 = x^2 + 2x + 1
x2+x−6=0x^2 + x - 6 = 0

3. Solve the quadratic equation

(x+3)(x−2)=0(x + 3)(x - 2) = 0
x=−3 or x=2x = -3 \text{ or } x = 2

4. Check for extraneous roots and state the final answer

Substitute x=−3x = -3 into the original equation:
LHS=−3+7−(−3)−1=4+3−1=4≠0\text{LHS} = \sqrt{-3+7} - (-3) - 1 = \sqrt{4} + 3 - 1 = 4 \neq 0
So x=−3x = -3 is rejected.

Substitute x=2x = 2 into the original equation:
LHS=2+7−2−1=9−3=0\text{LHS} = \sqrt{2+7} - 2 - 1 = \sqrt{9} - 3 = 0
So x=2x = 2 is the valid solution.

Answer

x=2x = 2

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