>>542

　∠B=90ﾟ ゆえ直角三角形で
　tan(C) = 3/4,
　tan(5C) = {(tan C)^5 -10(tan C)^3 +5(tan C)}/{1 -10(tan C)^2 +5(tan C)^4}
　　 = 237/(4･19･41),
　0 < 5C - 180ﾟ < tan(5C) = 237/(4･19･41) ≒ 4.3578625ﾟ,
　180ﾟ < 5C < 184.3578625ﾟ
　36ﾟ < C < 36.8715725ﾟ

(蛇足)
　sin(C) = 3/5,
　sin(5C) = 5(sin C) -20(sin C)^3 +16(sin C)^5 = -237/3125 ≒ -4.3453119ﾟ,
　4.3453119ﾟ < 5C - 180ﾟ,
　36.8690624ﾟ < C