Solve Arb 07Dc75
1. **State the problem:** We are given the equation $$(ar^b)^4 = 81^{24}$$ where $a$ and $b$ are positive integers. We need to find the values of $a$ and $b$.
2. **Rewrite the equation:** Using the power of a power rule, $$(ar^b)^4 = a^4 r^{4b}$$ so the equation becomes $$a^4 r^{4b} = 81^{24}$$.
3. **Express 81 as a power of a prime:** Note that $$81 = 3^4$$, so $$81^{24} = (3^4)^{24} = 3^{96}$$.
4. **Rewrite the equation with prime bases:** We want $$a^4 r^{4b} = 3^{96}$$.
5. **Assuming $a$ and $r$ are powers of 3:** Let $$a = 3^x$$ and $$r = 3^y$$ where $x,y$ are positive integers.
6. **Substitute into the equation:**
$$a^4 r^{4b} = (3^x)^4 (3^y)^{4b} = 3^{4x} 3^{4by} = 3^{4x + 4by}$$.
7. **Equate exponents:** Since $$3^{4x + 4by} = 3^{96}$$, we have
$$4x + 4by = 96$$.
8. **Simplify:** Divide both sides by 4:
$$x + by = 24$$.
9. **Find integer solutions:** We want positive integers $x,y,b$ such that $$x + b y = 24$$.
10. **Choose simplest values:** To find $a$ and $b$, we can set $y=1$ (so $r=3$), then
$$x + b = 24$$.
11. **Express $a$ and $b$:** Since $a = 3^x$ and $r = 3^1 = 3$, the original expression is
$$(a r^b)^4 = (3^x imes 3^b)^4 = 3^{4(x+b)} = 3^{96}$$ which matches if $$x + b = 24$$.
12. **Pick $a$ and $b$:** For simplicity, let $a = 3^{12}$ and $b = 12$ so that $$12 + 12 = 24$$.
13. **Final answer:**
$$a = 3^{12} = 531441$$
$$b = 12$$
Thus, $a = 531441$ and $b = 12$ satisfy the equation.