А)0,9x+0,4=13  б)(7+х)(5-х)=0

[latex]a) 0,9x + 0,4 = 13 \\ 0,9x = 13 - 0,4 \\ 0,9x = 12,6 \\ x = \frac{12,6}{0,9} =14 \\ x=14[/latex] [latex]b) (7+x)(5-x) = 0 \\ 35-7x + 5x - x^2 = 0 \\ -x^2-2x+35=0 \\ x^2 + 2x - 35 = 0 \\ D = 2^2 - 4 * (-35) = 4 - (-140) = 4 + 140 = 144 \\ x_1_,_2 = \frac{-2 б \sqrt{D} }{2} = \frac{-2 б \sqrt{144} }{2} = \frac{-2 б 12}{2} \\ x_1 = 5 \\ x_2 = -7[/latex]