<span>sin 300 <0;
</span>tg 200° >0;
cos 100° < 0;
⇒ <span>sin 300 tg 200 cos 100 <0 ( -)
</span><span>cos2 </span>≈ <span>cos(2*57,3</span>°<span>) </span>≈<span> cos 114</span>° < 0 (-)
.........................
An=(-1)^(n-1)*8
----------------------------------
79*81= <span> (80-1)(80+1)= 80</span>²-1²= 6400-1=6399