Solve using the Pythagorean identity
​

Solution given

Cos

consider Pythagorean theorem

Subtracting both side

doing square root on both side we get

Similarly

Substituting value of

we get

Since

In IVquadrant sin angle is negative

a4 - 3a2 - 1

step-by-step explanation:

step   1   :

equation at the end of step   1   :

  ((a4) -   3a2) -   1

step   2   :

trying to factor by splitting the middle term

2.1     factoring   a4-3a2-1  

the first term is,   a4   its coefficient is   1 .

the middle term is,   -3a2   its coefficient is   -3 .

the last term, "the constant", is   -1  

took the test it's 94