Lines and Angles — NCERT Solutions

Given: Lines AB and CD intersect at O. ∠AOC + ∠BOE = 70° and ∠BOD = 40°.

Step 1: Find ∠AOC.

Since AB is a straight line, ∠AOC and ∠BOC are supplementary... but more directly, ∠AOC and ∠BOD are vertically opposite angles (formed by intersecting lines AB and CD).

$$\angle AOC = \angle BOD = 40^\circ \quad (\text{Vertically opposite angles})$$

Step 2: Find ∠BOE.

We are given:
$$\angle AOC + \angle BOE = 70^\circ$$
$$40^\circ + \angle BOE = 70^\circ$$
$$\angle BOE = 70^\circ - 40^\circ = 30^\circ$$

Step 3: Find ∠COE.

Since AB is a straight line, the angles on one side of AB along line CD sum to 180°.

Ray OE lies between OB and OC (from the figure). The angles ∠BOE, ∠COE together with consideration of the straight line:

$$\angle BOC = 180^\circ - \angle AOC = 180^\circ - 40^\circ = 140^\circ \quad (\text{Linear pair, since AOB is a line})$$

Now, ∠COE = ∠BOC − ∠BOE:
$$\angle COE = 140^\circ - 30^\circ = 110^\circ$$

Step 4: Find reflex ∠COE.
$$\text{Reflex } \angle COE = 360^\circ - 110^\circ = 250^\circ$$

Answers: $\angle BOE = 30^\circ$ and reflex $\angle COE = 250^\circ$.

Given: Lines XY and MN intersect at O. Ray OP is such that ∠POY = 90°. The angles a and b are formed by ray OP with line MN, and a : b = 2 : 3.

Step 1: Express a and b.

From the figure, ray OP stands on line MN, so:
$$a + b = 180^\circ \quad (\text{Linear pair})$$

Let $a = 2k$ and $b = 3k$. Then:
$$2k + 3k = 180^\circ \implies 5k = 180^\circ \implies k = 36^\circ$$

So, $a = 72^\circ$ and $b = 108^\circ$.

Step 2: Find c.

From the figure, ∠POY = 90°. Since XY is a straight line:
$$b + \angle POY = 180^\circ - a \quad \text{... let us use the geometry carefully.}$$

Ray OP is between ray OX and ray OM (from the figure). We have:
$$\angle POY = 90^\circ$$

Since ∠MOY = b (angle between OM and OY) and ∠POY = 90°:
$$\angle POM = \angle POY - \angle MOY \quad \text{or} \quad \angle MOY = b$$

Actually, from the figure: $a$ is the angle ∠POM and $b$ is the angle ∠POX (or vice versa). Let us use:
$$a + \angle POY = 180^\circ \quad (\text{since MN is a line, angles on one side})$$

Wait — ray OP stands on line XY, so:
$$\angle POX + \angle POY = 180^\circ$$
$$\angle POX = 180^\circ - 90^\circ = 90^\circ$$

Now ray OM stands between OX and OP (from figure), giving:
$$a + b = \angle XON = 180^\circ \quad \text{(already used)}$$

From the figure, $b$ is between OP and OX:
$$b = \angle POX - a \implies 108^\circ = 90^\circ - a$$ — this doesn't work, so let us reconsider.

From the figure: ∠POY = 90°, and $a$ = ∠POM (between OP and OM, on the Y-side), $b$ = ∠MOX.

Then: $a + \angle POY = \angle MOY$ ...

Most standard interpretation: $a$ is ∠NOP and $b$ is ∠NOX (or ∠MOX), with MN as a line through O.

Standard solution:

Ray OP stands on line XY:
$$\angle XOP + \angle YOP = 180^\circ \implies \angle XOP = 90^\circ$$

Now, $a$ = ∠MOP and $b$ = ∠MOX, with $a + b = 180^\circ$ (MN is a line), $a:b = 2:3$, giving $a = 72^\circ$, $b = 108^\circ$.

Since $\angle XOP = 90^\circ$:
$$b = \angle MOX = 108^\circ$$

Now, $c$ = ∠NOY (vertically opposite to ∠MOX... no).

$c$ and $b$ are vertically opposite angles (∠NOY and ∠MOX are vertically opposite):
$$c = b = 108^\circ$$

But let us verify: ∠NOX = a = 72°, and ∠MOY = 72° (vertically opposite). ∠XOP = 90°, so ∠NOP = 90° − 72° = 18°...

Actually the cleanest standard answer: $c$ is the angle ∠NOY.
$$\angle MOX = b = 108^\circ$$
$$c = \angle NOY = \angle MOX = 108^\circ \quad (\text{vertically opposite angles})$$

Answer: $c = 108^\circ$.

Given: ∠PQR = ∠PRQ.

To Prove: ∠PQS = ∠PRT.

Proof:

From the figure, SQ is a straight line (S, Q, R, T are such that SQR and QRT are straight lines — ray QS and ray RT are opposite rays making straight lines with QR... actually from the figure, ST is a straight line passing through Q and R, so ∠PQS and ∠PQR are a linear pair, and ∠PRT and ∠PRQ are a linear pair).

Since ray QP stands on line ST:
$$\angle PQS + \angle PQR = 180^\circ \quad \text{...(1)} \quad (\text{Linear pair})$$

Since ray RP stands on line ST:
$$\angle PRT + \angle PRQ = 180^\circ \quad \text{...(2)} \quad (\text{Linear pair})$$

From (1) and (2):
$$\angle PQS + \angle PQR = \angle PRT + \angle PRQ$$

But it is given that $\angle PQR = \angle PRQ$.

Subtracting ∠PQR (= ∠PRQ) from both sides:
$$\angle PQS = \angle PRT$$

Hence proved.

Given: Rays OA, OB, OC, OD meet at point O such that $x + y = w + z$.

To Prove: AOB is a straight line.

Proof:

The sum of all angles around point O is 360°:
$$x + y + w + z = 360^\circ \quad \text{...(1)}$$

Given: $x + y = w + z$ ...(2)

From (1) and (2):
$$(x + y) + (x + y) = 360^\circ$$
$$2(x + y) = 360^\circ$$
$$x + y = 180^\circ$$

Now, $x = \angle AOC$ and $y = \angle BOC$ (from the figure), and ray OC stands between OA and OB.

$$\angle AOC + \angle BOC = 180^\circ$$

Since the adjacent angles ∠AOC and ∠BOC are supplementary (sum = 180°) and OC is a common ray, by the converse of the Linear Pair Axiom, AOB is a straight line.

Hence proved.

Given: POQ is a straight line. OR ⊥ PQ, so ∠ROP = ∠ROQ = 90°. Ray OS lies between rays OP and OR.

To Prove: $\displaystyle\angle ROS = \frac{1}{2}(\angle QOS - \angle POS)$.

Proof:

Since OR ⊥ PQ:
$$\angle ROP = 90^\circ \quad \text{and} \quad \angle ROQ = 90^\circ$$

Since OS lies between OP and OR:
$$\angle ROS + \angle POS = \angle ROP = 90^\circ \quad \text{...(1)}$$

Also, since POQ is a straight line and OS is a ray:
$$\angle QOS + \angle POS = 180^\circ \quad (\text{Linear pair}) \quad \text{...(2)}$$

From (2):
$$\angle QOS = 180^\circ - \angle POS$$

Now compute the RHS:
$$\frac{1}{2}(\angle QOS - \angle POS)$$

From (1): $\angle POS = 90^\circ - \angle ROS$.

Substitute into RHS:
$$\frac{1}{2}(\angle QOS - \angle POS)$$

Since $\angle QOS = \angle QOR + \angle ROS = 90^\circ + \angle ROS$:
$$\frac{1}{2}\bigl[(90^\circ + \angle ROS) - (90^\circ - \angle ROS)\bigr]$$
$$= \frac{1}{2}\bigl[90^\circ + \angle ROS - 90^\circ + \angle ROS\bigr]$$
$$= \frac{1}{2}\bigl[2\angle ROS\bigr]$$
$$= \angle ROS$$

$$\therefore \angle ROS = \frac{1}{2}(\angle QOS - \angle POS)$$

Hence proved.

Given: ∠XYZ = 64°. XY is produced to point P (so XYP is a straight line). Ray YQ bisects ∠ZYP.

Figure description: X–Y–P is a straight line. Ray YZ makes an angle of 64° with YX. Ray YQ is between YZ and YP, bisecting ∠ZYP.

Step 1: Find ∠ZYP.

Since XYP is a straight line:
$$\angle XYZ + \angle ZYP = 180^\circ \quad (\text{Linear pair})$$
$$64^\circ + \angle ZYP = 180^\circ$$
$$\angle ZYP = 116^\circ$$

Step 2: Find ∠ZYQ and ∠QYP.

Since YQ bisects ∠ZYP:
$$\angle ZYQ = \angle QYP = \frac{\angle ZYP}{2} = \frac{116^\circ}{2} = 58^\circ$$

Step 3: Find ∠XYQ.

$$\angle XYQ = \angle XYZ + \angle ZYQ = 64^\circ + 58^\circ = 122^\circ$$

Step 4: Find reflex ∠QYP.

$$\angle QYP = 58^\circ$$
$$\text{Reflex } \angle QYP = 360^\circ - 58^\circ = 302^\circ$$

Answers: $\angle XYQ = 122^\circ$ and reflex $\angle QYP = 302^\circ$.