EM-3094

Cosine rule

E Math · Secondary 4 · Hard · Cosine rule

\triangle PQR

PR = 7\text{ cm}

QR = 11\text{ cm}

and

PQ = 12\text{ cm}

.

Angle

PRS

is a straight line with

S

on the extension of

PR

beyond

R

.

Without using a calculator, find the exact value of

\cos \angle QRS

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