csinA=acosC => a/c = sinA/cosC
由正弦定理 a/c = sinA/sinC
∴ sinC =cosC => ∠C = π/4
∴ ∠A + ∠B = 3π/4 ==> ∠B = 3π/4 - ∠A
3sinA - cos(B+π/4)
= 3sinA - cos( 3π/4 - A +π/4)
= 3sinA + cosA
= √10*sin(A+θ)
其中 sinθ = √10/10；tanθ = 1/3
∵ 0< tanθ < √3/3
∴ 0 < θ < π/6
∠A 的取值范围是 (0,3π/4 )
因此 3sinA - cos(B+π/4) = √10*sin(A+θ) 的最大值为√10；
无法得出 A为直角的结论,只要 C= π/4,等式就成立；
A 可在(0,3π/4 )上任意取值.