Calculate the value of \( a^{-2}b^{4} \) when \( a = 2 \) and \( b = 3 \).

4. Si tenemos que \( A=\frac{5^{n+2}-5^{n+1}}{5^{n+1}} \) halle la suma de cifras de \( A^{2} \)

Simplify the following: (1) \( \frac{x-1}{x^{2}+3 x+2} \div\left[\frac{x-3}{x+1}-\frac{x-4}{x+2}\right] \) (2) \( \frac{\frac{6 x+5}{x-5}}{\frac{3 x+20}{x-5}+2} \) (3) \( \frac{a^{2}+\left(1-a^{2}\right)}{(1+a) a} \times \frac{(1+a) a}{a^{2}-\left(1-a^{2}\right)} \)

Example Solve the equation \( 2 x^{2}+4 x-10=0 \) with error precision 0.05

3) If \( 8 x+y i=i^{47}+24 \), where \( i^{2}=-1 \), then \( x-y= \) \( \begin{array}{llll}\text { (a) } 2 & \text { (b) } 4 & \text { (c) }-4 & \text { (d) } 25 \text {, }\end{array} \)

(8) If \( L^{2} \) and \( L \) are the two roots of the equation : \( x^{2}+a x+8=0 \), then \( a=\ldots \ldots \ldots \) \( \begin{array}{llll}\text { (a) }-6 & \text { (b) } 6 & \text { (c) } 4 & \text { (d) } 8\end{array} \)