38:
=>\(sinA=\dfrac{2\cdot sin\left(\dfrac{B+C}{2}\right)\cdot cos\left(\dfrac{B-C}{2}\right)}{2\cdot cos\left(\dfrac{B+C}{2}\right)\cdot cos\left(\dfrac{B-C}{2}\right)}=\dfrac{cos\left(\dfrac{A}{2}\right)}{sin\left(\dfrac{A}{2}\right)}\)
=>sin(A/2)=1/căn 2
=>1/2*góc A=45 độ
=>góc A=90 độ