化简式子 证明等式化简1.cos(90+a)+sin(180-a)-sin(180+a)-sin(-a)证明1．sin(

化简
-sin a+sin a-(-sin a)-(-sin a)
=2sin a
证明
1.原式=sin a*cos b+cos a*sin b/cos a*cos b
分子分母同除以cos a*cos b
=tan a+tan b
2.原式=(sin a*cos b+cos a*sin b)(cos a*coa b+sin a*sin b)
将式子乘开后合并同类项
=sin a*cos a+sin b*cos

cos(90+a)+sin(180-a)-sin(180+a)-sin(-a)=-sin(a)+2*sin(a)+sin(a)=2*sin(a)
sin(a+b)/cosacosb=(sinacosb+cosasinb)/cosacosb=tana+tanb
2sin(a+b)cos(a-b)=sin(2a)+sin(2b)=2sinacosa+2sinbcosb
sin(a+b)cos(a-b)=sinacosa+sinbcosb