tính, theo hằng đẳng thức: a) ((a+b)-c)^2 b) (5a-3b)(5a+3b) c) (3x+1)(3x-1) d) (5x^2-2)(5x^2+2) e) (x^2+2/5y)(x^2-2/5y) f) (x+y+z)(x+y-z)

$\begin{array}{l} a){\left( {a + b – c} \right)^2} = {\left( {a + b} \right)^2} + {c^2} – 2\left( {a + b} \right)c\\  = {a^2} + {b^2} + {c^2} + 2ab – 2bc – 2ca\\ b)\left( {5a – 3b} \right)\left( {5a + 3b} \right) = 25{a^2} – 9{b^2}\\ c)\left( {3x + 1} \right)\left( {3x – 1} \right) = 9{x^2} – 1\\ d)(5{x^2} – 2)(5{x^2} + 2) = 25{x^2} – 4\\ e)\left( {{x^2} + \dfrac{2}{5}y} \right)\left( {{x^2} – \dfrac{2}{5}y} \right) = {x^4} – \dfrac{4}{{25}}{y^2}\\ f)(x + y + z)\left( {x + y – z} \right) = {\left( {x + y} \right)^2} – {z^2} = {x^2} + {y^2} – {z^2} + 2xy \end{array}$