cos(-4分之派)的值为多少

已知一正弦交流电流i=sin
(314t-π/4）A,则该交流电的最大值为 1A ,有效值为 0.707A
,频率为 50Hz ,周期为0.02S,初相位为-π/4

y=sinx的单调增区间为[-π/2+2kπ,π/2+2kπ] k属于整数
单调减区间为[ π/2+2kπ ,3π/2+2kπ] k属于整数
令t=3x-π/4,Y=2sint的单调增区间为[-π/2+2kπ,π/2+2kπ]
即-π/2+2kπ≤3x-π/4≤π/2+2kπ
-π/12+2kπ/3≤x≤π/4+2kπ/3
Y=2sin(3x-π/4)的单调增区间为[-π/12+2kπ/3,π/4+2kπ/3] k属于整数
同理可得Y=2sin(3x-π/4)的单调减区间为[π/4+2kπ/3,7π/12+2kπ/3] k属于整数

∵tana=-4/3 ==> sina/cosa=-4/3
==> sina=-4/3cosa,
∴由sina+cosa=1得
(-4/3cosa)+cosa=1
==> cosa=9/25
∵a为第四象限的角,
∴cosa=3/5.
∴sina=-4/3cosa=-4/5.
故f(a)=（1-√2sin(2a-π/4))/cosa
=(1-√2(sin(2a)cos(π/4)-cos(2a)sin(π/4)))/cosa
=(1-sin(2a)+cos(2a))/cosa
=(1-2sinacosa+2cosa-1)/cosa
=(-2sinacosa+2cosa)/cosa
=2(cosa-sina)
=2(3/5+4/5)
=2*7/5
=14/5.

1 因为(Sinx)^2=(1-Cos2x)/2所以f(x)=sin平方(2x-4分之派)=(1-Cos2(2x-4分之派))/2=1/2-Cos(4x-2分之派))/2周期是T=2派/4=派/22.CosC=(a^2+b^2-c^2)/2ab 余弦定理将b=1.c=根号3.角c=3分之2派 代入上式就能解出a的值...

sin²(π/4-a)+cos²(π/4-a)=1
sin(π/4-a)=5/13
所以cos2a=sin(π/2-2a)
=2sin(π/4-a)cos(π/4-a)
=120/169
sin(π/4+a)
=cos[π/2-(π/4+a)]
=cos(π/4-a)
=12/13
所以原式=10/13

1、最小值显然为-根号2.递增区间为【kpi-3pi/8,kpi+pi/8】,递减区间为【kpi+pi/8,kpi+5pi/8】
2、x在区间[-8分之派,2分之派],那么2x-pi/4属于【-pi/2,3pi/4】,画图可知最小值为0,x取-pi/8；最大值为根号2,x取pi/8.
若有不懂,欢迎追问.

cos(-π/4)=√2/2