1.

Cos 25°cos35°-sin10°sin20°​

Answer»

-STEP EXPLANATION:COS 25°cos35°-sin10°sin20°={Cos(25+35)°+Cos(25-35)°}÷2-{Cos(10-20)-Cos(10+20)}÷2={Cos 60°+Cos(-10°)}÷2-{cos(-10°)-Cos(30°)}÷2=Cos 60°/2+Cos10°/2-Cos10°/2+Cos30°/2=(1/2)÷2+0+(√3/2)÷2=1/4+√3/4=(1+√3)/4



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