Moderate -0.8 This is a straightforward application of the Pythagorean identity sin²A + cos²A = 1, requiring only substitution, basic algebra, and correct sign selection for an obtuse angle. It's easier than average as it's a single-step problem with routine technique and no problem-solving insight needed.
Use of $\sin^2 x + \cos^2 x = 1$ | M1, A1 |
$\cos^2 x = \frac{24}{25}$ | M1, A1 |
selecting the negative value | M1, A1 |
$\cos x = -\frac{\sqrt{24}}{5}$ | | **Total: 4 marks**
3 Given that $A$ is the obtuse angle such that $\sin A = \frac { 1 } { 5 }$, find the exact value of $\cos A$.
\hfill \mbox{\textit{OCR MEI C2 Q3 [4]}}