# 2017 CBSE Class X

Maths Board Paper One mark Question Video Explanations

So, ∠APO = ∠BPO
Therefore, ∠APO = $$frac{∠APB}{2}$ = $\frac{60}{2}$ = 30o In right triangle OAP sin 30o = $\frac{\text{opposite side}}{\text{hypotenuse}}$ = $\frac{OA}{OP}$ OA is the radius = ‘a’ Therefore, sin 30o = $\frac{1}{2}$ = $\frac{a}{OP}$ Or the length of OP = 2a #### Question 4: What is the common difference of an A.P. in which a21 – a7 = 84? #### Video Explanation #### Explanatory Answer Key Data: a21 – a7 = 84. The numbers are in an AP. For an AP, the nth term an = a1 +$n – 1)d, where a1 is the first term of the AP and 'd' is its common difference.
So, a21 = a1 + (21 – 1)d = a1 + 20d and
a7 = a1 + (7 – 1)d = a1 + 6d

a21 – a7 = a1 + 20d – (a1 + 6d) = 84
Or 14d = 84
d = $\frac{84}{14}$ = 6