Сумму не скажу, промежуток не дан
<em>cos125=cos(180-55)=-cos55</em>
<em>cos85=cos(90-5)=sin5</em>
<span><em>-cos55cos5+sin55sin5=-(cos55cos5-sin55sin5)=-(cos60)=-1/2</em></span>
1/b*x^2=2*x-b
x^2=2*b*x-b^2
x^2-2*b*x+b^2=0
D=4*b^2-4*b^2=0
x=2*b/2=b
x=b
1)2√18=2*3√2=6√2
2)2√75=2*5√3=10√3
3)6√3*3√3=18*3=54
4)3√2*2√2=6*2=12