# easy trigonometric function

• Apr 27th 2010, 04:30 AM
mastermin346
easy trigonometric function
given that sec 60 degree=2 find the sec 20 degree without using a calculator..i know this is easy question,but i dont know how to get the answer..please help!
• May 5th 2010, 06:13 AM
Quote:

Originally Posted by mastermin346
given that sec 60 degree=2 find the sec 20 degree without using a calculator..i know this is easy question,but i dont know how to get the answer..please help!

this reply is a bit late but you might still be interested to know the solution (Wink) and it's not easy .

sec 60 =2

cos 60=1/2

Do you know the triple angle formula ,

cos 3x =4cos^3 x -3cos x

Substitute x=20 ,

cos 3(20)=4cos^3 20- 3cos 20

1/2=4cos^3 20-3 cos 20

8cos^3 20-6cos 20-1=0

Solving this ,

cos 20 = 0.94 , cos 20= -0.77(undefined) , cos 20 = -0.17 (undefined) --- since cos cant be -ve in the 1st quadrant

sec 20 = 1/0.94

sec 20 = 1.064
• May 5th 2010, 06:23 AM
mrmohamed
Quote:

this reply is a bit late but you might still be interested to know the solution (Wink) and it's not easy .

sec 60 =2

cos 60=1/2

Do you know the triple angle formula ,

cos 3x =4cos^3 x -3cos x

Substitute x=20 ,

cos 3(20)=4cos^3 20- 3cos 20

1/2=4cos^3 20-3 cos 20

8cos^3 20-6cos 20-1=0

Solving this ,

cos 20 = 0.94 , cos 20= -0.77(undefined) , cos 20 = 0.17 (undefined)

sec 20 = 1/0.94

sec 20 = 1.064

HI all
good work mathaddict but how u find it without calculator!!!!!
Quote:

given that sec 60 degree=2 find the sec 20 degree without using a calculator
mrmohamed