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<span>1) (-0,25 - 3/4 + 1/2) * (-0,2) + 3,9 = (-1/4 - 3/4 + 2/4) * (-1/5) + 39/10 = -2/4 * (-1/5) + 39/10 = 1/10 + 39/10 = 40/10 = 4
2) </span><span>(4,3-7,8)*(-5,6+8,2) = -3,5*2,6 = -9,1
</span>3) <span>4/15*(-5/8) = -20/120 = -1/6</span>
Пусть сторона а
(а+20)²-а²=3600⇒а²+40а+400-а²= 40а+400=3600 ⇒40а =3200 ⇒а=80