<span>ctga*ctgb+1/ctga*сtgb-1=(сosacosb+sinasinb)/</span><span>(сosacosb-sinasinb)</span>=
=cos(a-b)/cos(a+b)
2sin15° *(cos10° *cos5°-sin10° *sin5°)=2sin15° *cos(10°+5°)=
=2sin15° *cos15°=sin(2*15°)=sin30°=1/2=0,5
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