a) sinA khác 0
<=> sinA.sin(B-C)=$sin^2B-sin^2C$
<=>$sinA.sin(B-C)=4sin \frac{B+C}{2}.cos \frac{B-C}{2}.cos \frac{B+C}{2}.sin \frac{B-C}{2}$
<=>$sinA.sin(B-C)=sin(B+C).sin(B-C)$
A+B+C=180 độ =>sinA=sin(B+C) =>ĐPCM
b)https://diendan.hocmai.vn/threads/luong-giac.282558/
a) sinA khác 0
<=> sinA.sin(B-C)=$sin^2B-sin^2C$
<=>$sinA.sin(B-C)=4sin \frac{B+C}{2}.cos \frac{B-C}{2}.cos \frac{B+C}{2}.sin \frac{B-C}{2}$
<=>$sinA.sin(B-C)=sin(B+C).sin(B-C)$
A+B+C=180 độ =>sinA=sin(B+C) =>ĐPCM
b)https://diendan.hocmai.vn/threads/luong-giac.282558/