3. а) (3c+5d)² = б) (0,5 − 1,3n)² = в) (6f + 2,5z)² =

Используем формулы \((u+v)^2=u^2+2uv+v^2\) и \((u-v)^2=u^2-2uv+v^2\).

а) \((3c+5d)^2=(3c)^2+2\cdot3c\cdot5d+(5d)^2=9c^2+30cd+25d^2\).

б) \((0.5-1.3n)^2=(0.5)^2-2\cdot0.5\cdot1.3n+(1.3n)^2=0.25-1.3n+1.69n^2\).

в) \((6f+2.5z)^2=(6f)^2+2\cdot6f\cdot2.5z+(2.5z)^2=36f^2+30fz+6.25z^2\).