Question

超急!.已知△ABC中,a=8 b=7 B=60°,求边c及S△ABC.

Answer 1

1)

cosB=(a^2+c^2-b^2)/(2ac)

c^2-8c+15=0

c=3或c=5

S△ABC=1/2acsinB

S△ABC=6√3或10√3

2）2b=a+c

2sinb

=sina+sinc

=2sin(a+c)/2*cos(a-c)/2

=2sin(180-b)/2*cos(a-c)/2

2sin(b/2(cos(b/2)=cos(b/2)cos(a-c)/2

cos(a-c)/2=2sin(b/2)=1

(A-C)/2=0

A=C

等边三角形