\(A=cos^228+cos^241+cos^262+cos^249\)
\(=cos^2\left(28+41+62+49\right)\)
\(=cos^2360\)
\(=2\)
ta có :
\(A=\cos^228+\cos^241+\cos^262+\cos^249\)
\(A=\cos^228+\cos^241+\sin^228+\sin^241\)
\(A=1+1=2\)
chúc bn hc tốt
\(A=cos^228+cos^241+cos^262+cos^249\)
\(=cos^2\left(28+41+62+49\right)\)
\(=cos^2360\)
\(=2\)
ta có :
\(A=\cos^228+\cos^241+\cos^262+\cos^249\)
\(A=\cos^228+\cos^241+\sin^228+\sin^241\)
\(A=1+1=2\)
chúc bn hc tốt
1) Cho \(\cos a.\sin a=\frac{1}{5}\)Tính cot a
2) Chứng minh rằng
a)\(\frac{\cos a}{1-\sin a}=\frac{1+\sin a}{\cos a}\)
b)\(\frac{\left(\sin a+\cos a\right)^2-\left(\sin a-\cos a\right)^2}{\sin a.\cos a}=4\)
tính nhanh
A=\(sin^242^o+sin^243^o+sin^244^o+sin^245^0+sin^246^o+sin^247^o+sin^248^o\)
B=\(\cos^215^o-cos^225^o+cos^235^o-cos^245^o+cos^255^o-cos^265^o+cos^275^o\)
Cho tam giác nhọn ABC có các đường cao AH,BK,CL. CMR:
a, \(\dfrac{S_{AKL}}{S_{ABC}}= \dfrac{AL.AK}{AB.AC}=cos^{2}A\)
b, \(\dfrac{S_{HKL}}{S_{ABC}}=1-cos^{2}A-cos^2B-cos^2 C\)
tính
\(\cos^215-\cos^225+\cos^235-\cos^245+\cos^255-\cos^265\)
cho cos a = 3/4 tính sin a, tan a, cos a
Tính cos²25° - cos²12° - cos²78° + sin30° + cos²65° + 7cot45° Cho tana = 3. Tính sin, cos, tan
Chứng minh tan2a - sin2a .tan2a=(1-cos a)(1+cos a)
Tính giá trị biểu thức:
a) \(\sin^230^0-\sin^240^0-\sin^250^0+\sin^260^0\)
b) \(\cos^225^0-\cos^235^0+\cos^245^0-\cos^255^0+\cos^265^0\)
cho tam giác abc nhọn .CMR: cos A + cos B + cos C ≤ \(\frac{3}{2}\)