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Cos²x=1-sin²x
2(1-sin²x)-2sin²x=1
2-4sin²x=1
-4sin²x=-1
sin²x=1/4
x=+-π/6 +πn, n ∈Z
1)25y^2 - a^2 = (5y - a)(5y + a)
2)c^2 + 4bc + 4b^2 = ( c+2b)(c+2b)
6х-ху+30-5у=5х-ху+35-7у
х+2у= 5
10х-ху-40+4у=2х-10-ху+5у
8х-у=30
{х+2у=5
{8х-у=30. |*2
{х+2у=5
{16х-2у=60
__________
17х= 65
х=3 14/17
3 14/17+2у=5
2у=5-3 14/17
2у=1 3/17
у= 20/17*1/2
у=10/17
От: ( 3 14/17; 10/17)