[toán 10] chứng minh hệ thức lượng!

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nttthn_97

a) $sinB.cosC+sinC.cosB=sin(B+C)=sin(\pi-A)=sinA$

b) $b.cosC+c.cosB=b.\dfrac{a^2+b^2-c^2}{2ab}+c.\dfrac{a^2+c^2-b^2}{2ac}=a$

c) $a.R.sinB.sinC=a.R.\dfrac{b}{2R}.\dfrac{c}{2R}= \dfrac{abc}{4R}=S$
 
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