Solve Maclaurin Expansion Equation: cos(x)-2x2=0

[pat666]

Homework Statement

Use the first two terms of the Maclaurin series expansion of cos(x) to solve the equation cos(x)-2x²=0 . Check its accuracy with a calculator ( is in radians).

Homework Equations

f(x)=f(0)+x(f'(0)) first two terms

The Attempt at a Solution

So I have found the Maclaurin expansion of cos(x) to be 1. This seems ridiculous to me so I'm wondering if its correct?
then:
1-2x²=0 so x=+-(sqrt(2)/2)
=+-.239755

I'm not sure what to say about the accuracy??
FYI this is an exam practice Q
Thanks

 

[fzero]

You want the first two non-vanishing terms. Go to higher order in the expansion.

 

[pat666]

ok, non vanishing means not 0?

 

[pat666]

now I get cos(x)=1-x^2/2
and the solution is +-0.816497
the actual answer is +-.63456 - seems reasonably accurate?