$3.450 A=\cos ^{6}\left(\frac{5 \pi}{2}+\frac{1}{2} \arcsin \frac{3}{5}\right)+\cos ^{6}\left(\frac{7 \pi}{2}-\frac{1}{2} \arcsin \frac{4}{5}\right)$

$3.450 A=\cos ^{6}\left(\frac{5 \pi}{2}+\frac{1}{2} \arcsin \frac{3}{5}\right)+\cos ^{6}\left(\frac{7 \pi}{2}-\frac{1}{2} \arcsin \frac{4}{5}\right)$

The text above is already in a mathematical expression format, which is the same in both Chinese and English. Therefore, no translation is needed for the mathematical content itself. However, if you need the text "将上面的文本翻译成英文，请保留源文本的换行和格式，直接输出翻译结果。" translated, it would be:

"Translate the text above into English, please retain the original text's line breaks and format, and output the translation directly."
## Solution.

Let $\alpha=\arcsin \frac{3}{5}, \beta=\arcsin \frac{4}{5}$, then $\cos \alpha=\frac{4}{5}, \cos \beta=\frac{3}{5} \Rightarrow$ $\Rightarrow A=\sin ^{6} \frac{\alpha}{2}+\sin ^{6} \frac{\beta}{2}=\left(\frac{1-\cos \alpha}{2}\right)^{3}+\left(\frac{1-\cos \beta}{2}\right)^{3}=0.009$.

Answer: $A=0.009$.

## $3.451 A=\cos 260^{\circ} \cdot \sin 130^{\circ} \cdot \cos 160^{\circ}$.

## Solution.

$A=\cos 80^{\circ} \cdot \cos 40^{\circ} \cdot \cos 20^{\circ}=\frac{8 \sin 20^{\circ}}{8 \sin 20^{\circ}} \cdot \cos 20^{\circ} \cdot \cos 40^{\circ} \cdot \cos 80^{\circ}=\frac{\sin 160^{\circ}}{8 \sin 20^{\circ}}=\frac{1}{8}$.

Answer: $A=\frac{1}{8}$.