sin^2 20+cos^2 50+sin20cos50=?
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sin^2 20+cos^2 50+sin20cos50=?
sin^2 20+cos^2 50+sin20cos50
=(sin20+cos50)^2-sin20cos50
=(sin20+sin40)^2-sin20cos50
=(2sin30*cos(-10))^2-sin20cos50
=cos10^2-sin20cos50
=(1+cos20)/2-1/2*[sin70+sin(-30)]
=1/2+1/2*cos20-1/2*[cos20+sin(-30)]
=1/2+1/4
=3/4
这里要用到三个三角函数公式:
sina+sinb=2sin((a+b)/2)cos((a-b)/2
(cosa)^2 =1/2+1/2*cos2a
cos(a)cos(b)=1/2*[cos(a+b)+cos(a-b)]
ok.
=(sin20+cos50)^2-sin20cos50
=(sin20+sin40)^2-sin20cos50
=(2sin30*cos(-10))^2-sin20cos50
=cos10^2-sin20cos50
=(1+cos20)/2-1/2*[sin70+sin(-30)]
=1/2+1/2*cos20-1/2*[cos20+sin(-30)]
=1/2+1/4
=3/4
这里要用到三个三角函数公式:
sina+sinb=2sin((a+b)/2)cos((a-b)/2
(cosa)^2 =1/2+1/2*cos2a
cos(a)cos(b)=1/2*[cos(a+b)+cos(a-b)]
ok.
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