The maximum value is √ (1+a? )

When x=-π/8, f(x) has a maximum value.

So f (-π/8) = sin (-π/4)+acos (-π/4) = √ (1+a? )

-√2/2+a*(√2/2)= √( 1+a？ )

Square on both sides

( 1/2)(a- 1)？ = 1+a？

Answer? -2a+ 1=2( 1+a？ )

Answer? +2a+ 1=0

(a+ 1)？ =0

a=- 1

0 & lttanA * tanB & lt 1

sinAcosA/cosAcosB>。 0，sinAcosA & gt0,∴cosacosb>； 0

Sina cosa/cosa cosb & lt； 1, and both parties take cosAcosB, cosa cosb-Sina cosa & gt；; 0，cos(A+B)>0,∴cosc<； 0

Must be an obtuse triangle.