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1.(1+2u)(1-2u)(1-4u²)=(1-4u²)(1-4u²)=(1-4u²)²

2.(2 3/4)²-(1 1/4)²=(2 3/4-1 1/4)(2 3/4+1 1/4)=1 2/4* 4=1.5*4=6

3. $\frac{(16-25x^4}{4+5x^2} = \frac{(4^2-5^2(x^2)^2)}{4+5x^2} = \frac{(4^2-(5x^2)^2)}{4+5x^2} = \\ = \frac{(4-5x^2)(4+5x^2)}{4+5x^2} = 4-5x^2$

4. $( \frac{2a^{-1}}{3b^{-2}})*12ab^5 =( \frac{3b^{2}}{2a})*12ab^5 =\frac{3b^{2}}{a}*6ab^5 = \\ = \frac{3b^{2}}{1}*6b^5 =18b^7$