198․ա)cos(π/4-α)=(cosα+sinα)√2/2
բ)cos(π/4+α)=(cosα-sinα)√2/2
գ)sin(π/4-α)=(cos-sinα)√2/2
դ)sin(π/4+α)=(cosα+sinα)√2/2
199.ա)α=15º
√2(-√3-1)/4 , √2(-√3+1)/4 , 2-√3 , 2+√3
բ)α=75º
√2(-√3+1)/4 , √2(-√3-1)/4 , 2+√3 , 2-√3
203․ա)sin27ºcos3º+cos27ºsin3º=sin(27+3)=1/2
բ)cos87ºcos27º+sin87ºsin27º=1/2
գ)cos92°cos28°-sin92°sin28°=-1/2
դ)sin105°cos45°-cos105°sin45°=√3/2