Rewrite using identity: Use the identity sin2(θ)=21−cos(2θ) to rewrite each term.sin2(10)=21−cos(20)sin2(70)=21−cos(140)sin2(20)=21−cos(40)sin2(80)=21−cos(160)
Substitute into equation: Substitute the expressions back into the original equation.(1−cos(20))/2−(1−cos(140))/2−(1−cos(40))/2+(1−cos(160))/2
Combine like terms: Combine like terms.(21−21−21+21)−(cos(20)/2−cos(140)/2−cos(40)/2+cos(160)/2)
Simplify constants and cosine terms: Simplify the constant terms and combine the cosine terms. 0−(cos(20)/2−cos(140)/2−cos(40)/2+cos(160)/2)
Use cosine angle sum identities: Use the cosine angle sum identities, cos(180−θ)=−cos(θ) and cos(θ)=cos(−θ), to rewrite cos(140) and cos(160).−cos(20)/2+cos(40)/2−cos(20)/2
Combine cosine terms: Combine the cosine terms. −cos(20)/2−cos(20)/2+cos(40)/2
Final simplification: Simplify the expression. −cos(20)+2cos(40)