Question

sin69°-sin3°+sin39°-sin33°

Answer 1

sin(a+b)=sinacosb+sinbcosa

sin(a-b)=sinacosb-sinbcosa

相加得到[sin(a-b)+sin(a+b)]=2sinacosb

相减得到[sin(a+b)-sin(a-b)]=2sinbcosa

由此推算出来的

sinx+siny=2sin[(x+y)/2]cos[(x-y)/2]