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$1) \ 4t^2+28t+49 = (2t)^2+7*2*2t+7^2 = (2t+7)^2\\\\ 2) \ 0.04t^2-0.4tx+x^2 = (0.2t)^2-2*0.2tx + x^2 = (0.2t - x)^2\\\\ 3) \ 0.25t^2 - 3ty+9y^2 =\\\\ (0.5t)^2-0.5*2*3ty + (3y)^2 = (0.5t - 3y)^2\\\\ 4) \ \frac{4}{25}y^4+y^2t^3+\frac{25}{16}t^6 =\\\\ (\frac{2}{5}y^2)^2 + 2*\frac{2}{5}*\frac{5}{4}y^2t^3 + (\frac{5}{4}t^3)^2 =(\frac{2}{5}y^2 - \frac{5}{4}t^3)^2$

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