更新时间2021-06-13 10:38:57
a/sinA = 1/sin(120°) = 2/√3;
sinC = sin( 60° - B ) = sin60°cosB - cos60°sinB = (√3/2)cosB - (sinB)/2
正弦定理,b = (2/√3)sinB,c = (2/√3)[ (√3/2)cosB - (sinB)/2 ] = cosB - (sinB)/√3;
y = 2b + 3c = (4/√3)sinB + 3cosB - (3/√3)sinB = (sinB)/√3 + 3cosB;
y' = (cosB)/√3 - 3sinB = 0,驻点 cosB = 3√3sinB;
y'' = (-sinB)/√3 - 3sinB < 0,驻点是极大点;
故当 cosB = 3√3sinB 时,2b + 3c = (sinB)/√3 + 3cosB 有最大值;
(cosB)^2 = 27(sinB)^2,1 - (cosB)^2 = 1 - 27(sinB)^2 = (sinB)^2,
sinB = 1/(2√7),cosB = 3√3/(2√7)
2b + 3c 最大值为 (sinB)/√3 + 3cosB = 1/(2√21) + 9√3/(2√7) = (2√21)/3 。