<span>cos(π/5)*cos(2π/5)</span><span><span>=<span><span>sin(π/5)*cos(π/5)*cos(2π/5)/</span><span>sin(π/5)</span></span></span><span>=<span><span>sin(2π/5)*cos(2π/5)/</span><span>2sin(π/5)</span></span></span><span>=<span><span>sin(4π/5)/</span><span>4sin(π/5)</span></span></span><span>=<span>1/4</span></span></span>
<span>cos(A+B) = cos A cos B - sin A sin B
</span><span>cos(A-B) = cos A cos B + sin A sin B</span>
2sinasinb + cosacosb - sinasinb = cosacosb + sinasinb = cos(a - b)
5*4/12+7=5/3+7=5/3+21/3=26/3=8⅔
4*6/9+2/3=8/3+2/3=10/3=3⅓
(x-3)²>x(x-6)
x²-6x+9>x²-6x
9>0 верно
Решение на фото, которое прикреплено