5)cos5°-2(cos(25°-20°)-cos(25°+20°))/2=
=cos5°-cos5°+cos45°=cos45°=1/√2;
6.a)cos44°cos16°=(cos(44°-16°)+cos(44°+16°))/2=
=0,5cos28°+0,5cos60=0,5cos28°+1/4;
b)cos59°cos31°=(cos(59°-31°)+cos(59°+31°))/2=
=0,5cos28°+0,5cos90=0,5cos28°+0;
a-b)0,5cos28°+1/4-0,5cos28°=1/4;
1/4=1/4;
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Answers & Comments
5)cos5°-2(cos(25°-20°)-cos(25°+20°))/2=
=cos5°-cos5°+cos45°=cos45°=1/√2;
6.a)cos44°cos16°=(cos(44°-16°)+cos(44°+16°))/2=
=0,5cos28°+0,5cos60=0,5cos28°+1/4;
b)cos59°cos31°=(cos(59°-31°)+cos(59°+31°))/2=
=0,5cos28°+0,5cos90=0,5cos28°+0;
a-b)0,5cos28°+1/4-0,5cos28°=1/4;
1/4=1/4;