<span>sin2a+sin4a=2sin3a*cosa</span>
<span><span>cos2a cosa=1/2 (cosa+cos3a)</span></span>
<span><span>2sin3a*cosa<span>+sin6a=4 sin3a*1/2 (cosa+cos3a)</span></span></span>
<span><span><span>2sin3a*cosa+sin6a=2sin3a cosa+2sin3a*cos3a</span></span></span>
<span><span><span>sin6a=2sin3a*cos3a</span></span></span>
X+y=81
x-y=15
x=81-y
81-y-y=15
x=81-y
-2y=-66
x=81-y
y=33
x=48
1/4х3+3у2 при х=-2 и у=-1
1/4*(-2)*3=2.5
3*(-1)*2=-6
2.5+(-6)=-3.5