已知sin(a+兀/4)=3/5,且a属于(兀/2,3兀/4),则sina=?
问题描述:
已知sin(a+兀/4)=3/5,且a属于(兀/2,3兀/4),则sina=?
答
sin(a+π/4)=3/5sinacosπ/4+cosasinπ/4=3/5√2/2(sina+cosa)=3/5sina+cosa=3√2/5cosa=3√2/5-sinacos^2a=18/25-6√2/5sina+sin^2a1-sin^2a=18/25-6√2/5sina+sin^2asin^2a-3√2/5sina-7/50=0(sina+√2/10)(sina-7...