[กขฃคฅฆง]

31.

$A+B = 90^o$

$sinA = cosB$

$2cos^2A+cosB = 2(1-sin^2A)+cosB = 2(1-cos^2A)+cosB = -2cos^2B+cosB+2$

ให้ $-2cos^2B+cosB+2 = k$

$2cos^2B-cosB+k-2 = 0$