a,A=(sin3°)^2+(sin15°)^2+(sin75°)^2+(sin87°)^2

A=(sin3°)^2+(sin15°)^2+(cos15°)^2+(cos3°)^2

A=((sin3°)^2+(cos3°)^2)+((sin15°)^2+(cos15°)^2)

A=1+1

A=2

b,B=cos0°+ cos20°+cos40°+...+cos160°+cos180°

B=cos0°+(cos20°+cos160°)+(cos40°+cos140°)+(cos60°+cos120°)+(cos80°+cos100°)+cos180°

B=1+(cos20°-cos20°)+(cos40°-cos40°)+(cos60°-cos60°)+(cos80°-cos80°)+(-1)

B=1+0+0+0+0+(-1)

B=0

c,C=tan5°tan10°tan15°...tan80°tan85°

C=(tan5°tan85°)×(tan10°tan80°)×(tan15°tan75°)×(tan20°tan70°)×(tan25°tan65°)×(tan30°tan60°)×(tan35°tan55°)×(tan40°tan50°)×tan45

C=(tan5°cot5°)×(tan10°cot10°)×(tan15°cot15°)×(tan20°cot20°)×(tan25°cot25°)×(tan30°×cot30°)×(tan35°cot35°)×(tan40°cot40°)×tan45°

C=1×1×1×1×1×1×1×1×1

C=1

a,A=a^2sin90°+b^2cos90°+c^2cos180°

A=a^2×1+b^2×0+c^2×(-1)

A=a^2-c^2

b,B=3-(sin90°)^2+2×(cos60°)^2-3(tan45°)^2

B=3-1^2+2×(1/2)^2-3×1^2

B=3-1+2×(1/4)-3

B=2+1/2-3

B=-(1/2)

c,C=(sin45°)^2-2(sin50°)^2+3(cos45°)^2-2(sin40°)^2+4tan55°×tan35°

C=((sin45°)^2+3(cos45°)^2)-2((sin50°)^2+(sin40°)^2)+4×(tan55°×tan35°)

C=((sin45°)^2+3(cos45°)^2)-2((sin50°)^2+(cos50°)^2)+4×(tan55°×cot55°)

C=((sin45°)^2+3(cos45°)^2)-2×1+4

C=(căn(2)/2)^2+3(căn(2)/2)^2)+2

C=1/2+3/2+2

C=2+2

C=4