17/07/08 03:52:44.12 E7CWjLAg.net
>>53
sin(x) + cos(x) = y とおく。
1 - sin(x)^5 - cos(x)^5
= (1/2) {1-sin(x)} {1-cos(x)} F(sin(x)+cos(x))
= (1/4) (1-y)^2 F(y)
≧0,
F(y) = 4+3y+2yy+y^3 ≧ 8 - 5√2 > 0,
1 + sin(x)^5 + cos(x)^5
= (1/2) {1+sin(x)} {1+cos(x)} F(sin(x)+cos(x))
= (1/4) (1+y)^2 F(-y),
≧ 0,
F(-y) = 4-3y+2yy-y^3 ≧ F(√2) = 8 - 5√2 > 0,
を使うとか。