更新时间2018-07-08 00:03:58
急需!!
3乘以根号3除以4
2bsinB+2csinC=bc+√3a
2bsin[180°-(A+C)]+2csinC=bc+√3a
2bsin(A+C)+2csinC=bc+√3a
A+C=90°,即C=30°,B=90°
即2b+c=bc+√3a
SABC最大=1/2√3ac=1/2(2b+c-bc)=1/2【2b+c(1-b)】
即ABC越接近直角三角形,面积越大。
已知∠A=π/3,2bsinB+2csinC=bc+根号下3a,求三角形ABC面积的最大值
2bsinB+2csinC=bc+√3a
bc=2bsinB+2csinC-2asinA
2RsinBsinC=2sin²B+2sin²C-2sin²A
2RsinBsinC=2sin²B+2sin²C-2(sinBcosC+cosBsinC)²
2RsinBsinC=2sin²B+2sin²C-2sin²Bcos²C-2cos²Bsin²C-4sinBcosBsinCcosC
2RsinBsinC=2sin²B(1-cos²C)+2sin²C(1-cos²B)-4sinBcosBsinCcosC
2RsinBsinC=4sin²Bsin²C-4sinBcosBsinCcosC
sinBsinC(4sinBsinC-4cosBcosC-2R)=0
∵sinBsinC≠0→2R=-cos(B+C)=cosA=½
S=½bcsinA=¼√3bc=¼√3(2R)²sinBsin(⅔π-B)
f(B)=sinBsin(⅔π-B)
f'(B)=cosBsin(⅔π-B)-sinBcos(⅔π-B)=sin(2B-⅔π)→驻点B=⅓π
当三角形为正三角形时,面积最大
Smax=¼√3(½)²·¾=3√3/64
2bsinB+2csinC=bc+√3a
bc=2bsinB+2csinC-2asinA
2RsinBsinC=2sin²B+2sin²C-2sin²A
2RsinBsinC=2sin²B+2sin²C-2(sinBcosC+cosBsinC)²
2RsinBsinC=2sin²B+2sin²C-2sin²Bcos²C-2cos²Bsin²C-4sinBcosBsinCcosC
2RsinBsinC=2sin²B(1-cos²C)+2sin²C(1-cos²B)-4sinBcosBsinCcosC
2RsinBsinC=4sin²Bsin²C-4sinBcosBsinCcosC
sinBsinC(4sinBsinC-4cosBcosC-2R)=0
∵sinBsinC≠0→2R=-cos(B+C)=cosA=½
S=½bcsinA=¼√3bc=¼√3(2R)²sinBsin(⅔π-B)
f(B)=sinBsin(⅔π-B)
f'(B)=cosBsin(⅔π-B)-sinBcos(⅔π-B)=sin(2B-⅔π)→驻点B=⅓π
当三角形为正三角形时,面积最大
Smax=¼√3(½)²·¾=3√3/64