1) cos69=cos (90-21)=sin21;
cos79=sin11; cos10+cos11cos21+sin21sin11=cos10+cos (21-11)=2cos10;
2) sin20+sin13cos33-sin33cos13=sin20+sin (13-33)=0;
2a²-12ab+18b²+23ab=2a²+11ab+18b²
a=3√3 b=√3
2*27+11*3√3*√3+18*3=54+99+54=207
Решение во вложении
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А) (√10+5) / (2+√10) =[ (√5)*(√2) + (√5)^2] / [(√2)^2 + (√2)*(√5)] = = [ (√5)*(√2 + √5)] / [ (√2)*(√2 + √5) = (√5) / (√2) = √(5/2)
Б) (а - 3√а) (/2√а - 6) = [(<span>√a)^2 - 3√a)] / 2*(√a - 3)] = </span><span>= [(√a)*(√a - 3)] / [2*(√a - 3)] = √a/2</span>