Question

数学高二 。。19题

数学高二 。。19题19

Answer 1

请采纳

追问

还有一道

Answer 2

如图，看不懂可以画个草图

Answer 3

19.
(1)
A=3π/4, b=√2c
A=3π/4
=>B+C =π/4
cos(B+C)=√2/2
cosB.cosC - sinB.sinC = √2/2
cosB.cosC - (bsinC/c).sinC = √2/2
cosB.cosC - (b/c) = √2/2
cosB.cosC - √2 = √2/2
cosB.cosC = 3√2/2
(2)
S△ABC=2
(1/2)bcsinA =1
(1/2)bcsin(3π/4) =1
(1/2)b(b/√2) .(√2/2) =1
b^2 =4
b=2
Also
(1/2)bcsinA =1
(1/2)(√2c).c .(√2/2) =1
c^2 =2
c=√2
By cosine-rule
a^2=b^2+c^2-2bc.cosA
=4 + 2 +8(2)(√2)(√2/2)
= 4+2+16
=24
a=2√6