解
AB=√(cos70-cos10)^2+(sin70-sin10)^2
=√cos^2 70+cos^2 10-2cos10cos70+sin^2 70+sin^2 10-2sin10sin70
=√2-2cos10cos70-2sin10sin70
=√2-2(cos10cos70+sin10sin70)
=√2-2×cos(70-10)
=√2-2×1/2
=√2-1
=1