Math(Please check)
posted by Abby .
1) Use the cofunction identites to evaluate the expression without the aid of a calculator.
cos^2 18 degrees + cos^2 72 degrees.

Error

The cofunction identities that are useful here are
cos(90x) = sinx and
sin(90x) = cosx
and the fact that cos(AB) = cosAcosB + sinAsinB
so cos^2 72 = (cos72)(cos72) = (sin18)(sin18)
then your
cos^2 18 degrees + cos^2 72 degrees
= cos18cos18 + sin18sin18
= cos(1818)
= cos 0
= 1
Respond to this Question
Similar Questions

Trig
What is the exact value of cos 40 degrees cos 50 degrees minus sin 40 degrees sin 50 degrees do you know the formula cos(A+b) = cosAcosB  sinAsinB now compare it with your cos 40 degrees cos 50 degrees minus sin 40 degrees sin 50 … 
Math(Please check)
1) Use the cofunction identites to evaluate the expression without the aid of a calculator. cos^2 18 degrees + cos^2 72 degrees. Would I use the forumula cos(pi/2  Q) = sin Q 
Trigonometry
Please review and tell me if i did something wrong. Find the following functions correct to five decimal places: a. sin 22degrees 43' b. cos 44degrees 56' c. sin 49degrees 17' d. tan 11degrees 37' e. sin 79degrees 23'30' f. cot 19degrees … 
trig
Evaluate sin(29 degrees)cos(31 degrees) + cos(29 degrees)sin(31 degrees) exactly, without using a calculator. 
trig
Assume that we know sin(18 degrees)=(sqrt(5)1)/4, cos(18 degrees)=(sqrt(sqrt(5)+5))/8, find sin(42 degrees) exactly, without using a calculator. Also find cos(42 degrees). 
maths
evaluate without using a calculator, leaving your answer as surds (a)sin105 degrees (b)tan15 degrees (c)cos 75 degrees 
PreCalc
Evaluate the expression without using a calculator(show work please)? 
Trigonometry  LONESTAR
Simplifying steps without using the calculator for: tan(cos^1(1/10)) cos(sin^−1(1/x)) Assume x is positive tan(cos^−1(12/13)) cos^−1(cos 150°) This is pretty much the entire section we are doing. My teacher is … 
Please help Math
a. Use the formulas for lowering powers to rewrite the expression in terms of the first power of cosine: (cos^4)x (or cos*x to the 4th power) b.Use an appropriate Half Angle Formula to Find the Exact value of the expression: 1. tan … 
Math
Find the exact value of cos 1 degree + cos 2 degrees + cos 3 degrees + ... + cos 357 + cos 358 degrees + cos 359 degrees.