Ctg1=tg89
ctg2=tg88
ctg3=tg87
............................
lgtg1+lgtg2+lgtg3+......+lgctg87+lgctg88+lgctg89=
lg(tg1*tg2*tg3*....*ctg3*ctg2*ctg1)=lg1=0
<span> ((-1)</span><span>^2)</span><span>^5-1^2=0
<span>(-1)^7+(-1)^4=0</span></span>
<span>(3a-b)(a+b)+(b-3a)(b+3a)=</span><span>(3a-b)(a+b)-(3a-b)(b+3a)=</span>
=(3a-b)(a+b-b-3a)=(3a-b)(-2a)=-2a(3a-b)=-6a²+2ab<span>
</span>
(3x+2)^2-(3x-1)^2=(3x+2+3x-1)(3x+2-3x+1)=(6x+1)*3=18x+3