Cos5*cos55*cos65=? полное решение, ! дано всё в градусах (а не в радианах)

Cos5°*cos55°*cos65°  * *   cosα*cosβ =(1/2)(cos(α+β) +cos(α -β.  произведен.  в сумму  * * * * * cos(180° -α) = - cosα _формула * * * * * * cosα/2 =  ±  √(1+cosα)/2   _формула половинного угла  * *  * cos5°cos55°cos65°    =(1/2)(cos60°  +cos50°)cos65°  =(1/2)(1/2+cos50°)cos65° =(1/4)cos65°  +(1/2)cos50°cos65°  =(1/4)cos65°  +(1/4)(cos115°+cos15°) = (1/4)(cos65°  +cos115°+cos15°) =(1/4)(cos65°  +cos(180°-  65°) +cos15°)  = (1/4)(cos65°  -cos 65° +cos15°)  =(1/4)cos15°   =(1/4)√((1+cos30°)/2)   = (1/4)√((1+√3/2)/2)=(1/4)√((2+√3)/4) =(1/8)√(2+√3)   = (1/8√2)√(4+2√3) =(2+√3)/8√2 = √2(2+√3)/16.

Сos5*1/2(cos(55-65)+cos(55+65))=cos5*1/2(cos10+cos120)= =1/2*cos5(cos10-cos60)=1/2*cos5*(cos10-1/2)=1/2*cos5cos10-1/4*cos5= =1/2*1/2(cos(5-10)+cos(5+/4cos5=1/4*(cos5+cos15)-1/4cos5= =1/4*cos5+1/2*cos15-1/4*cos5=1/4*cos15

№1. а)8х+12у=4(2х+3у);

б)21а+28у=7(3а+4у).

№2. а)3b²-3b=3b(3b-1);

б)а⁴+2а²=а²(а²+2).

№3. а) х³-3х²-х=х(х²-3х-1);

б)2m⁶-4m³+6m=2m(m⁵-2m²+3).