Ответ: <span>4)1,5</span>
=cos(a)*cos(a)-(-sin(a))*sin(a)=cos^2(a)+sin^2(a)=1
(7х-у=0.
(3х-у=-12.
(у=7х.
(3х-7х=-12.
(у=7х.
(-4х=-12.
(у=7х.
(х=3.
(у=7*3.
(х=3.
(у=21.
(х=3.
Ответ: (3;21)
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20x-20=80x;⇒20x-80x=20;⇒-60x=20;
x=-20/60=-2/3;